ISP chips are very very very interesting in most of applications I'm thinking. But sometimes I would use some other chips for their special feature. But, I can't affort to put an external EPROM for the code, the system should have embedded flash for the purpose I've explained in previous posts. Then if we have a uC with an external flash, a way to have a kind of IAP would allow to use these chips as if they were ISP.
To make a non-ISP chip do ISP, you need the code in (moved to) another memory than what you are programming.
3 possibilities exist 1)program in RAM (1-2k), 2)program in external flash and 3) a chip with paged flash. Solution 3) seems to get a lot of people in trouble, I have not tried it. For 1) and 2) you would typically be better off handling the "special feature" (e. g. a/d) in a chip made for that purpose in combination with an ISP uC than making a uC with the "special feature" do ISP.

Erik

PS
you mention some awful prices, are they quoted at 30k pieces, if so ask your distributor what he charges for swampland in Florida.