Branson 8510 here. 5 gallon tank with six transducers. Picked it up on ebay for around $200. To give you an idea on size, a 2.9 lower fits in it without issue. There's photos of it on my build thread.
Don't skimp on an ultrasonic or the solution. I usually run Branson MC3. Stuff works wonders and doesn't hurt aluminum or brass.
It's the second biggest time saver I own. Cleaning handguns means I just remove wooden parts and throw them in. 5 gallon tank means it's easy to drop Mason jars in with task specific cleaner, or really dirty parts to save the solution.
I can go on for hours about ultrasonics, but theres really only two to buy... Branson and Omegasonics.
You will NEVER regret spending the money on either.