I'm a guitarist that would like to get into bass more.
Currently I need a bass amp that is cheap, good quality and will be able to hold up with a singer, 100w bass amp, and a drummer.
Maybe around 300USD?
You're asking a lot for $300.00; particularly the part about keeping up with a 100-watt guitar amp. What kind of cabinet does he/she have? 100 watts through a 4x12 is going to require some serious firepower for you to keep up.

Look at what is available used. Peavey's Tour 450 and 700 heads can often be found at very good used prices. Put one on top of a 2x10 or 1x15 cabinet. If your guitarist has a 4x12, then you'll need a 4x10 to keep up. Mic'ing the whole thing can compensate, but at most clubs and similar venues you'll be mic'ing only the vocals and maybe a monitor mix or two.
"Maybe this world is another planet's hell?" - Aldous Huxley