I'm going to guess car conditions aren't equal, but he can say for certain (maybe the donor Riviera is in rough shape?).
Rivieras are kinda slick looking, bit they're somewhat tanks compared to W-body's. There's also alot less availability of upgrades and replacement parts for the rest of the vehicle outside the engine bay, due to being a short-run platform in low volume without much following. Heck, even GP/Regal/Monte/Impala guys have minimal options, especially over 10yrs since production ended, but still a massive amount of options in comparison.
If you love Riv's, I get it, but it's going to cost a bit to keep one in good shape and have it hold the power of any upgrades using the 4T65e (or even trying to swap an 80e, but that's an electrical undertaking most here aren't trying to get into, due to limited info/interest).