Part of the low compensation was that Arizona was likely going to release him as well as the fact that Simmons was in the 4th year of his contract (base salary of $1.01M) with Arizona having declined his 5th-year option due to cost of $12.7M (uncertain if Giants can pick that up or not)
"According to Pro Football Focus, he's played 432 snaps on the defensive line, 1,077 in the box, 63 at free safety, 641 in the slot, 65 at corner and 432 as a wide rusher. The 6-foot-4, 238-pounder has appeared in 50 games with 37 starts, and his regular season totals include 258 career tackles, 13 tackles for loss, 11 quarterback hits, four interceptions (one returned for a touchdown), 16 pass breakups, seven forced fumbles, and 7.5 sacks."
Simmons is actually coming off his best season so far in spite of missing 4 games so for the Giants, it is a low cost high reward trade for them and with Martindale running a 3-4 that also plays a lot of nickel/dime type defense Simmons has the athleticism to be a quality addition for the Giants.