There are about 1000 keyboards and 400 modules that have been made in the modern era --- that is, since midi came into common use.
Everything from the incredible flexiblility - and the price to match - of the Roland V-synth to the Motif rack series featuring MLAN --- Yamaha's 'won't give up on it' subsitute for midi to the Korg Triton TR's much beloved of the hiphop crowd, there's no shortage of options out there.
New modules have become less common in recent years, but rack versions of older machines should be thick on the ground on eBay and Craig's List.
One that I lusted after in years past that I've seen listed used for around $300 is the rack version of the Yamaha CS6x. It has a sampler and a bunch of cool features and is easy to program.