I found this for you. See
http://www.astroanarchy.com.au/eyepieces.html
You cannot get any cheaper than this to get started. Either a pair of 25mm Kellner for $5 each or Sky-Watcher Super eyepiece for $10 each. Either would be good in your telescope with the Kellner looking like better build quality. The Skywatcher feels like it’s made of plastic but it works OK. (My son has one with his ST80). The Kellner will have a narrower apparent field of about 40 deg.
There’s lots of other eyepieces below but to get started you can get away with 2x$5 + $15 postage.