OP says "RF" and you got my attention. but then say that you want a few tens of kHz? Can you share more about the application and what kind of waveforms you require? The architecture (and price) really depends on the type of waveforms you want to generate. As @simon43mentioned, you could build or buy one for a few dollars, but you will be limited to the basic config. If you want something more flexible that uses a DAC, you could generate any arbitrary waveform including multiple tones, noise, modulated tones, the list goes on...