Now, I have primarily been shopping the AUGs and have run across several things. First the price seems to vary quite a lot, from as little as $211 to about $300. Also there seems to be some variation in the descriptions about the FPS that it will project love down range.
It doesnt look like anyone answered this so I'll try to do my best. In terms of price most online retailers will vary based on if they are in Hong Kong, or in the US, ofcourse when you order from HK its usually more expensive... In terms of the FPS most online retailers that are well... Less trustworthy will say the the guns they feature are higher fps than they really are, whether they use .12 rounds or something of the sort to crono it or just do whatever they can to get that little boost in fps for their guns. If you want to know real FPS' Dusty at jungletoy should be able to tell you the actual fps on almost any gun on the market
In terms of reliable retailers I've personally shopped w/ WGC, and airsoft atlanta both are great sites. I'm the kind of person who likes to buy everything at stores I can walk into so the majority of everything I have came from Jungletoy.
As for the CA m4 I own one and I run a 120 spring w/ a MATCHED 9.6v battery and I have had no problems so far...
just my $.02