Can someone help me with price advice?

[Marrish]

Hello, I just got this bike. I paid 50 bucks which I thought was fair. It is a Giant Acapulco probably from the late 90s/early 2000s. The bike functioned well until today when I felt the pedal wobbling and the pedal fell off. So I was reading a few other forums and they said that the crank arm was most likely the problem. The nut also appears to be stripped. So my question is should I just go to a bike shop and get a new nut or should I just order a crank arm set off of ebay. Also if I order a crank arm off ebay which one should I get. I am looking to spend under 30 bucks for this since I am a broke college student who has a commutes 10 miles each day. Currently it has the stock crank arm.

[Painkiller]

try to post pics of what you have, i may have a spare arm or another crankset. $30 is slim budget for something decent. plus Bottom bracket may also need replaced because they are sized to the type of crankset that will be used.