What's the most I should pay for a lightly used FN SPR A5M?

The A5m’s were going for 2200ish new if I recall. The A1 and A2’s were the ones that had the cheaper deals. I’d say anywhere on either side of 1500 your good. If you don’t have to have the McMillan A5 and can settle for an A3 or A4 then you should be able to find deals in the 1100 range.
 
Yeah thinking about it, Hondo is right; the a5 did command a bit of a premium. I have the a3 and I think I paid around $1600-1700 new back in 08
 
Seller says the rifle has less than fifty rounds fired. I was looking on gun broker and it looks like maybe used SPRs run in the $1500 range

If it the current A5M with DBM then those sell new usually for $2499-$2699 depending who you buy from. If it’s nice and low round count, anything sub $2000 for the A5M is a great deal.
If it’s an A1/A2 SPR then that’s different from the A5M significantly. The A1/A2’s new we’re around $1500 new and used $900-$1100
 
^^^^ This. A5M XP is the top of the SPR line (and I think the only one still in production). I've got a new one sitting in the safe I paid $2460 for three years ago. Lowest new I'm seeing is $2300 for 24" and $2000 for 20".