$300 can go pretty far for a used setup. But new not far at all, especially if you need everything to make a complete setup filters, heaters, decor. In my area a 125 tank only can be had for about $200 new plus, tops 40, plus lights $75 - $100.
If you search craiglist enough and can drive say 2 hours in any directions you'd be surprised what you can find. A lot of full setup 125s by me for $250 - $350. Just throw out offer the worst they can say is no, look for tanks than have been listed for a long time, most times they get to a pointed of just wanting to get rid of it and will take whatever they can get.
Also remember you can build a stand for a tank to save money.