In that case, any 1333MHz SO-DIMM will work a treat.* 8GB is pretty reasonable at ~$50, but if you wanted to splurge, a 16GB kit can be had for under $400 these days (and likely to plummet below $200 by the end of the year, if previous trends are anything to go by).
You'd get $14 for your RAM, but I presume you'd have to post it in, cutting the savings.
*Cheap stuff is a bit riskier, but should still be okay.