I never equated the two. Of course you're right and the mere fact of quantity doesn't have anything at all to do with quality. But Rioja has been producing wine for export for centuries and it's got huge bodegas that consistently produce wine of better quality than even many small, artisanal producers. Riscal's reserva is actually even ageworthy - I've had them over 30 years back and they're under $20.
The fact is that there just aren't all that many wines produced in Jumilla that are all that good and the producers there are well aware of that. Moreover, they're all pretty new to the export market. There has obviously been wine in the area for centuries, but it's only in the 1990s, after being inspired by Priorat, that much really started happening in Jumilla. Prior to that, it was bulk wine country and mostly still is. They got DO status only in 1996 based on the efforts of a tiny group.
Casa Castillo started making exportable wines only in the 1990s and they're usually recognized as one of the first "names" in Jumilla. One single importer contracted with Casa de la Ermita, Casa Castillo and Finca Luzón and that was pretty much how Jumilla was introduced to America in the late 1990s.
Carchelo only started up in the 1980s, maybe even 1990, don't remember, and Olivares started making exportable wine in the 1990s. Luzón is also an older bodega that only recently starte making exportable wine. Juan Gil too was an old wine family but they only started making their own wine in 2001 or 2002. Alceño from Luis Martinez only started up in 1990 or 2000, and I'm not sure if it's imported right now or not.
Who else would you recommend under $20?
Again, I'm not knocking the wines themselves and the producers are smart to make something they can export. But there's no where nearly the number of good drinkable wines under $20 that one finds in Rioja.