I had a under $200 eagle on my RIB, mono screen, and it worked famously.
If all you need is an inexpensive finder, they work great. I am partial to the Lowrance line as have had and heard good reports of product performance and factory service from them. Lowrance gets higher marks from user reports for sonar. Spending more gives you bigger screens, great depth, color display, and more detail options.
Personally I like a transom 'ducer because they are cheap, work good, and have the temp sensor. Mount them on a piece of starboard on your transom rather than screw into your hull.
There's a X50 mono thru Cabellas for $119. Comes with the 'ducer, has temp read too. The X52 is just a little bigger. Hummingbird is fine too.