Well... it's pretty common to see 240GB SSDs for ~$80 (or roughly $0.35 per GB) brand new these days, sometimes less if combined with a visa checkout / masterpass / paypal promo. I snagged a couple 240GBs for ~$60 during the black friday sales, and a 120GB for $35.
Normally you take 30-40% off 'new' pricing for used gear. Particularly with SSDs, since they have finite lifespans - may be helpful to post how much usage these have on em. The M4s are a few generations old now as well, so that's another factor to consider.