Here is a list to compare engine prices but GM is still best source
GM Performance - JEGS High Performance and GM will have tranny info as well. A word or caution on newer Auto's ... make sure car computer interface does not become an issue. I have always found a car with an automatic tranny appeals to a MUCH smaller market and build cost is higher. I am a big believer in building whatever works for you. Most of my builds use a GM engine and that niche market works for me but an Auto really limits MY resale options.