As stated above, knowledgeable buyers want and/or expect hi-caps for any handgun model introduced pre-ban, even if the gun itself was made post-ban. But a buyer is not going to want to pay current used market prices for the full package. Assume you have an XYZ 9mm with 2 factory 10-rd mags plus 2 factory hi-caps. If the average market price for a used XYZ with two post-ban mags is $350, and the average retail (Web, gun show, etc) price for a hi-cap is $100, you are going to find it very difficult to get $550 for the package (sticker shock). And you know what a dealer will offer you for the whole thing. A buyer is going to want a discount on the package.
I have noticed a lot of guns offered for sale, on this forum and elsewhere, where the initial listing is a package like the one described above. Then a few days or a few weeks later, the listing is changed to offer only the gun and the post-bans. The package hasn't sold, and the seller has sold or is going to sell the hi-caps individually. There just seems to be a psychological block (or an economic one) against biting the bullet (little gun joke there) on the whole package at once. Even though the guy that buys the gun with post-bans is going to buy some high-caps later, and you and he know it. It just seems so much cheaper to pass on the hi-caps for now. It is a lot easier to justify just one hi-cap for $100 a few weeks after you have the gun. And then one more at next month's gun show.
Just over the last six months, I have noticed prices for factory hi-caps for "big guns" (SIG, Glock, etc) are creeping up. Supply and demand: supply is fixed, demand is growing. As this trend continues, it will probably be even harder to sell a total package with a big price tag. But you will be able to get higher prices for each hi-cap, if you can find retail buyers. The high caps should be easier to sell on the Net, since they don't require an FFL. (This assumes that the supply of hi-cap mags is in fact fixed, and that they are not being counterfeited in off-shore sweat shops and smuggled into the country. At $100 and rising, the druggies are going to start hiding hi-caps inside bales of marijuana. The mags will be worth more per ounce than the grass.)