For <= $500 there are some nice options out there. Most of the rifles from the major vendors will work just fine and give decent to excellent accuracy. I've bought Savage, Rem, and Weatherby bolt guns and not been disappointed with any of them. Stevens and Mossberg also have gotten a lot of good press from owners. You may want to hold on to the 243 even if you are going to go with a 270 as a) a guy's first rifle has great sentimental value when he gets older b) never hurts to have a back-up and c) few people complain that they have too many guns

I'd not rule out a 7-08 or 308 either as they are comparable to the 270 and work well. Price for a given rifle in new or used condition, hmmmm, depends on your geographic region and how much competition there is among sellers. I always check the on-line auctions and local stores and that helps me figure out what a reasonable baseline is for price.
Depends a bit though on your boy and how much growin' he is going to do and what fits him best. I'd recommend you take him around to some of the shops and shoulder different models and let him get an idea of which one feels best to him, nothing like an ill fitting rifle to turn a lad off from shooting with heavier calibers. Regarding glass, there are a lot of good options out there and having a good scope makes a rig even more pleasant to shoot.
Good luck and have fun, getting a new rifle and scope is always a fun task and while there are many models to choose from, it is better to have a feast rather than a famine when it comes to choices.