I'm not exactly sure djpaintles point. In the case of any AR price is always relative to something else. The 6920 is the benchmark standard rifle and right now they are cheaper than they have been in the past. To answer the question posed the 516 is a decent AR for a first time AR. Should you purchase it really depends on what fairly cheap is. I'd buy one every time versus the alternatives at $499 (DPMS territory), but I'd never even think about buying one at $699 since a 6920 would be so close in price. Hopefully that helps.
“So in war, the way is to avoid what is strong, and strike at what is weak.”
I know you already have your answer but my Sig 516 has been a great rifle. I bought it many years ago for a steal when a vendor was clearing out extra inventory. I have 4 local friends who also bought them at the same time and every one has been rock solid. It has always functioned perfectly for me with a wide variety of ammo and it has good accuracy using the Federal Champion 55 grain bulk pack from Walmart. I think it's a great first AR since it is easier to keep clean and is less lube sensitive than a direct impingement AR. The adjustable gas regulator is an excellent feature for firing suppressed.
The downsides already mentioned are weight and a proprietary rail, so it has a few limitations on aftermarket options compared to a standard DI AR15. For a first AR15 I would buy one and shoot the heck out of it as-is to learn the fundamentals.
I've got the 7.5. If you go 7.5, keep in mind she likes the heavier, stouter, boo-lits. Just sayin' yo.
It is a heavy platform, as has been stated. But the trade off is you only need to wave a mild dust rag over the gun for a brief moment to clean it. So there's that.
Everything considered, a suppressor ready, piston driven, AR is hard to beat as a first AR.