I'm with Sadie Kate on this one. I hate to say this, but I think someone's trying to take advantage of you. I doubt very much that you need to replace much of anything at 2700 miles, and I'm surprised that you've already gone through multiple chains and brake pads. Assuming you're regularly cleaning your drivetrain, you should get more life out of it than they're leading you to believe.

If I were you, I'd think twice about patronizing a shop that tried to sell me a bill of goods. I have multiple shops to select from in my area and I have learned over time (and sometimes the hard way) which shops--and which mechanics in each shop--to trust.

In the very least, I'd challenge them a bit on what they're telling you. With that in mind, it might be worth your while to gain some understanding of what to look for in terms of wear with respect to your bike's components so that you can exercise some independent judgment as to whether it's time to replace or rebuild something. I've just been doing some research myself on that here and on www.roadbikereview.com, and there's a lot of helpful information. You just need to do a few searches and a bit of reading.....

Good luck.