"Professional Sounds." I know what you mean, but I guess, in my mind, I can see a three year old at the piano and the producer saying 'press *that* key a 'little' harder.' NOT professional.

In a sense though, that isn't too far off target. However, it isn't the people who play the notes, but more like the people who record and produce the sounds. A Yamaha piano is still a Yamaha piano, but if you do a 'pro' recording session versus something less, the sounds will be different.
The fact of the matter is that to get the better sounds, specifically 'sample/playback', you're going to have to spend more money. That often includes larger RAM or SSDs (Solid State Drives) which tend to be more expensive, the storage for those sounds in a ROM. Then, for something like a keyboard, the playback system, etc.
Matt brings up Real Tracks, which is my opinion, if you're putting together backing tracks on the fly as a 'generic track', then those work very well, and of course, aren't samples, but real musicians playing. Hence, the sound from that is very pro, considering the people playing the stuff. But, if you're looking for the riff from Peter Gunn or The Pink Panther, you won't find it there. So, everything has it's place.
My opinion is that Bang For The Buck, right now the SD2 is one of the best options. You can look at Yamaha's Motif Rack, some of Korg's stuff, Roland has some stuff, and Ketron also offers a couple of other things.
Gary