128 GB is a small SSD indeed. If you plan to buy one, consider at least 240 GB. Upgrading 120 to 240 is only about 10 US dollars.
Unlike an HDD, SSDs have limited writes before cells wear out. 240 GB ones have about twice TBW (terabytes written), which means that you can wrtie 2 times more data over their life. Also, check the MTBF (Mean Time Between Failure). Some have 1 million hours, some have 2, maybe even more.
Opt for SSDs that have features like garbage collection, wear leveling, S.M.A.R.T and TRIM support. Also is good to get one with a DRAM chip (the ones with cache)
About the price, I couldn't find anything less than $40 to have the upper mentioned features.
If not sure what to pick, go for a Samsung 860 Evo or Crucial MX500. Both have 5 year warranty; while 860 evo has 150 TBW, MX500 has only 100, but 860 evo is about $65, while MX500 only $50.
Update: 860 evo might actually be also $50, but with in my country the cheapest is $60. Lookes on Amazon and was $57. Anyways, both should be just fine