You didn't mention whether or not you have a mic stand or if you needed vocals, but for $250 this is what I would do:

1. Do not get an omni for the very reasons that Nicole mentions. With a small amp behind you, you're sure to get feedback from an omni.

2. Get a Shure 58 for the vocals (around $99 with discount) and a Shure 57 for the instruments (around $77 with discount). Also pick up a stand adapter which clamps to the side of the mic stand and mounts the second mic at waist height (around $15). Remember to add in the cost of mic cables ($10 for 25 footers). You may want a small 6" gooseneck to top off your mic stand, for the vocal mic (or use a small boom). The Crate 30 has two inputs, but I can't remember if both inputs are XLR or not. If not, get an XLR(F) to a 1/4"(M) adapter for the second input ($10).

3. Remember that, in any case, you will need to stand behind the amp or again have feedback problems. (Put it on a low stand in front or just to the side of you, but in front of the mics). When you step up for your cut, you will have a vocal at mouth height with an instrument mic at the waist.

And you'll be under your $250 limit.

If you are really on a budget, get a mic stand top adapter for 2-3 mics, then get two Radio Shack 3001s (about $35 on sale) and mount them in a 45 degree arc, again behind the amp. These are actually very good mics (made by Shure) and come with a 15 foot XLR to 1/4 adapter cord which will go directly into any of the inputs on your amp.

But if your group is any good, and you're not playing cowboy bar mitzvahs, you're going to have to think about geting a better sound system than this. Good luck!