No idea, but I expect the creator would be looking for moderately serious money, unless of coruse you just bought a backing track of siutable length & sound and then added your own vocal as a separate track. This would be the simplest way.
Or since you are in Reaper, generate your own backing track and then add your own vocal. Either you talking/singing or someone who is already good at it and prepared to do it free or cheap.
Plenty of voice-over talent out there. FWIW I used to do jingles when I lived in Nashville & there were a ton of us desperate for work at the time!