Well, there really is no such thing as an 'ukulele amp, but you can use an acoustic guitar amp. Most of the battery powered amps that you are seeing (like the Roland Micro Cube and Vox) are fairly cheap. But when you start looking for something that plugs into the wall, you are looking at more power (watts) and that equals $$$. But spending that extra money gets you a better product.
There are no definite answers in this field, so you will have to do your research and try some different amps out.
A non-micro acoustic amp is probably going to start at the 30 watt range. If you are going to be playing in a band you will want around 100 watts. Just FYI, more watts = more power and loudness. And also headroom so you don't get any distortion - clearer sound. Beyond 100 watts and you might as well plug into a PA system.
I haven't tried as many acoustic amps as I would like, but here are my opinions. I have a Kamaka with a passive Hilo soundboard transducer pickup.
The best amp I have tried so far is the Roland AC-90. 90 watts, effects, mic and line channels for $700 something. Just really sweet. If I were to buy an amp today, that's what I would buy.
The Fishman loudboxes are okay. I didn't like the Fishman Solo amp. The reverbs sounded like they were screaming after the notes went away.
The Fender Acoustasonic sounded quacky with too much mid-range and fed back.
There are plenty to check out, but the Roland is on the top of my list. It's also expensive.
http://www.musiciansfriend.com/navigation?q=acoustic+amps
Other amps that will work with 'ukuleles are Keyboard and Bass amps. These will sound better than an electric guitar amp any day. Most of these are just little PA systems and they don't mess with the signal.
Something else to think about is a preamp. A lot of the acoustic amps have these built in, but you can get an outboard unit. I have the Fishman Prefix Pro EQ. Everyone else swears by the LR Baggs Paracoustic DI (direct-in box). I see that Baggs has a new pedal out though. The ad said it was a preamp plus tuner, etc...
Try, try, try. It's the only way to know what you like. And remember that you can buy used too.