There are tons of cheap USB oscilloscopes on ebay but I think the old saying "You get what you pay for" is very true in this case. I have bought 2 USB scopes off ebay. The first of which was around $100. I fried that one. Never figured out exactly what I did to fry it but it no worky any more. The second scope I bought was around $80 and it still works but the included software was absolute garbage. One of the biggest limitations with these cheap USB scopes I've noticed is the sample rate. The first scope I bought was advertised at 200kHz but that was total, not per channel. So, if I was taking 4 measurements then the max sample rate I'd see would be 50 kHz. I'd pay close attention to the sample frequency when choosing a scope. If you're signal is 24kHz and your scope is 500 kHz for example, then you'd see about 20 data points per period. It'll be up to you to decide if that resolution suits your needs. Also, there are bench top scopes that can output to a computer so you do have other options than a USB scope for taking screen shots. And, there are hand held portable scopes too. $700 - $800 is a good budget. You should be able to find something that works for you.