Amanda, that speaker would work fine as a monitor, but you'll need to get power to it, which may end up costing you more than a simple powered speaker.
I, too, am in the market for a small-venue system, and I've already started my 'wish list' of pieces to buy. So far, my total for the system itself (not counting the CDGs) is around $2500.
Another thought in the powered mixer department is the box-style mixer-amplifiers, like this one:
http://www.musiciansfriend.com/product/ ... sku=634246
I worked with quite a few karaoke companies that started with something like this. And sometimes the bigger musical outlets have great prices on packaged deals, as mentioned in the posts above.
And finally, if your budget is tight, remember that you don't
have to have a monitor right at first. Buy it later down the road, as your venues get bigger! :)