ok, so lately ive been getting into trap shooting, and my dad's been letting me use his Savage 12ga side-by-side. ive been looking for a 12ga for myself to use for this and probably nothing else since i dont hunt or anything. problem is, im poor. so i dont want to spend more than $150 or so, but im not aversed at all to buying a used gun (actually im almost damned sure thats what ill get). i like the side-by-side and i was thinking about even just a single-shot since you only get one shot at a time in trap (and these seem to be cheaper).

any suggestions? thanks in advance

Used 870 express with a 30" barrel (28" if you are under 5'-6").  

Then when you get the chance, try to pick up a High comb butt stock that will raise the POI of the shot over the bead, and have a middle bed installed.

But, in the mean time, you can add a filler to the comb with cardboard and tap. Ideally, you want the shotgun to shoot about 12" to 18" higher than the front bead.  This allows you to keep the bead under the target to see and track the target, and compensates for the target climbing up out of the box (read you pick the target up as it clears the front of the house, and crush it before it every makes it out past the 25 yard mark).

A used Remington 870 is a good recommendation. There are a lot of shooters out there busting up a lot of trap targets with them. Stick with a longer barrel like 28-30".

Eventually you may want to get a specialized trap gun. Trap guns are typically set up to shoot higher than your point of aim. This is because all trap targets are rising. With a normal shotgun you usually have to cover up the target to hit it, which can be a bit unnerving. But thats what I have to do since I use a field gun.