Which User gets the best abilities for the cheapest price
Nothing like the high end killers,but I still run an aka JJ Watt,because his x factor starts on and he can be tough to stop on the edge. I had the Freeney UL card and his abilities were fantastic as far as AP wise,but I didn't get nearly the production out of him that I would have thought,anyways.
I believe it is widely regarded (or was, until recently) that this Ryan Neal card is potentially the best USER available, although he doesn't meet your "cheapest price" requirement, as the card was an LTD and quite popular.
Basically, you want someone fast, preferably with ball skills so you can not only break up pass plays, but hopefully get a few picks from time to time. Having height is a HUGE advantage, both on offense and defense, which means you would probably be searching for this Julius Peppers, who is 6'7" can get a decent boost to speed with the new Sugar Rush Strategy item, making him rather viable again, although YMMV. He can be found for well under ~200k these days, but the market has gone wild lately due to the Easter promo drop(s), folks are stocking up on Training in anticipation of egg-rerolls, i believe.
Broadly speaking, Safeties tend to work better at the SUBLB position(s) and many/most folks typically try to hold down the middle of the field -- you'd be amazed how great you can do on defense if you are able to take away the opponents first read, that's it. Concentrate on that type of obvious pass defense in the middle and hope your pass rush gets home, so the QB isn't holding the ball all day. Very few folks run actual linebackers at SUBLB. Hopefully that helps.
DON'T be a "me" as i'm struggling to follow my own (or rather, the community's) good advice. I'm still determined to make an actual linebacker work as my USER card in the middle, i've stuck with Fred Warner, who has 99 Speed and all the top level stats we'd expect or want, plus 6'3" height (which is quite decent, but not amazing), 0AP Pick Artist & 0AP Enforcer... although i've also been tinkering with Poyer and Woodson as SUBLB2 this week, leaning towards Woodson for the moment, probably going to allow the AI to operate big Fred.