The question about whether Congress can single out abortion for special taxation reminds me of this item I wrote about taxes on guns, in my Implementing the Right To Keep and Bear Arms for Self-Defense: An Analytical Framework and a Research Agenda; the analysis for taxes on abortion should be pretty similar. (Note, incidentally, that the gun tax question would arise as to state taxes under the 40+ state constitutional individual rights to bear arms in self-defense, quite apart from the various Second Amendment debates.)
Taxes on guns and ammunition, or gun controls that raise the price of guns and ammunition, would be substantial burdens if they materially raised the cost of armed self-defense. A $600 tax proposed by Cook, Ludwig & Samaha [in another article in the same symposium for which my article was written -EV], justified by an assertion that "keeping a handgun in the home is associated with at least $600 per year in externalities," is one such example. "The poorly financed [self-defense] of little people," like their "poorly financed causes," deserves constitutional protection as much as the self-defense of those who can afford technologically sophisticated new devices or high new taxes. (See Martin v. City of Struthers, 319 U.S. 141, 146 (1943) (striking down ban on door-to-door solicitation, partly on the grounds that "[d]oor to door distribution of circulars is essential to the poorly financed causes of little people"); see also City of Ladue v. Gilleo, 512 U.S. 43, 56 (1994) (striking down ban on display of signs at one's home, partly on the grounds that "[r]esidential signs are an unusually cheap and convenient form of communication. Especially for persons of modest means or limited mobility, a yard or window sign may have no practical substitute.").) This is true whether the tax or expensive control is imposed on gun owners directly, or on gun sellers or manufacturers, just as a restriction on abortion can be a substantial burden even if it's imposed on doctors and not on the women who are getting the abortions.
High gun taxes should remain presumptively impermissible even if they are based on some (doubtless controversially calculated) estimate of the public costs imposed by the average handgun: The average takes into account both the very low cost stemming from guns that are always properly used by their owners, and the very high cost stemming from guns that are used in crime. The law-abiding owners thus are not just being required to "internalize the full social costs of their choices," even if you take into account as a "cost" the possibility that any gun will be stolen by a criminal. They are also being required to internalize the social costs of choices made by criminal users of other guns -- much as if, for instance, all speakers were charged a tax that would be used to compensate those libeled by a small subset of speakers.
Nonetheless, some modest taxes might not amount to substantial burdens, as a review of taxes and fees on other constitutional rights illustrates. Taxes based on the content of speech are unconstitutional, regardless of their magnitude. But this is a special case of the principle that discrimination based on certain kinds of characteristics -- race, sex, religiosity, or the content or viewpoint of speech -- is unconstitutional. Setting aside these special areas of constitutionally forbidden discrimination, and setting aside poll taxes, which were constitutional until the Twenty-Fourth Amendment forbade them, other kinds of taxes, fees, and indirect costs imposed on the exercise of constitutional rights are often permissible.
The government may require modest content-neutral fees for demonstration permits or charitable fundraising permits, at least if the fees are tailored to defraying the costs of administering constitutionally permissible regulatory regimes. The same is true for marriage license fees and filing fees for political candidates (though the Court has held that the right to run for office is protected by the First Amendment). The same is doubtless true of costs involved in getting permits to build on your own property, a right protected by the Takings Clause.
Likewise, regulations of the right to abortion are not rendered unconstitutional simply because they increase the cost of an abortion. The Court so held when upholding a 24-hour waiting period even though it required some women in states with very few abortion providers to stay in a hotel overnight or miss a day of work, and when upholding viability testing requirements that might have marginally increased the cost of an abortion. So long as the extra costs don't amount to "substantial obstacle[s]" to a woman's getting an abortion, they are constitutional.
At the same time, when a cost is high enough to impose a substantial obstacle to the exercise of a right for a considerable number of people, it is unconstitutional. This is likely also true when a cost goes materially beyond the cost of administering the otherwise permissible regulatory scheme, as several federal circuit court cases hold and some U.S. Supreme Court cases suggest. And if a law substantially burdens rightholders who are relatively poor, an exemption would likely be constitutionally required, as it has been with regard to permit fees for speakers and candidates.
I acknowledge that any such regime necessarily creates linedrawing problems and poses the danger that a genuinely substantial burden will be missed by judges who are deciding how much is too much. But, first, there is ample precedent for such tolerance for modest fees in other constitutional rights contexts, and it seems neither likely nor normatively appealing for the courts to conclude that the right to bear arms is more protected than these other rights. Second, the caselaw from those other areas can provide guideposts for the linedrawing process. And third, the caselaw from those other areas (as well as the general logic of the substantial burden threshold) can provide justification for a constitutional requirement that poor applicants be exempted from fees -- say, fees that dramatically increase the cost of a new gun, or that are required for periodic reregistration of an old gun -- that are substantial for them even if relatively minor for others.