You get what you pay for. Technically an acoustic amp will be able to take a.guitar and keyboard as well.
But if you can up your budget, go for the Hartke KM60, it has a nice 7-band graphic EQ along with 4 channels of input at the back and 1 microphone input in the front.
So you get a total of 3 instrument channels + 1 mic
or
4 instrument channels.
And that's like $450, as compared to your budget, you get a lot more.