Reserva in spain is minimum 3 year aging (oak cask + bottle, of which minimum 1 year in oak) before sale.  However, Rioja has a tighter requirement than than the requirement for spain in general, and I don't know the exact periods.

The long aging is not necessary expensive.  There is no requirement to be new oak, and I don't think it even specific it to be barrel (they only use the work "cask").  The major cost is thus just space and tides up capital.  You can get Rioja Reserva in HK easily for just $200 or even less.

EUR50 is very expensive for a Rioja Reserva, and the sales should give additional information to support why it derserve this price, either reviews by famous critics or a free tasting.