Hi Cathy,
I have used the spa at the Princesa Parc on several occasions and would have no problem recommending it, you are correct that there are better spa's elsewhere in Andorra (Caldea, Andorra la Vella and Sport Wellness, Soldeu) however if you don't want to have the hassle of travelling elsewhere then the Princesa Parc is your best option.
Just to give you a rough idea of prices:
Two hour entry into the Wellness Centre (Hydromassage pool, 2 jacuzzis, sauna, hot marbles, hamman, hydromassage showers, foot bath, relaxation area etc..) = €12
25 min massage (relaxation, sports, lymphatic drainage, foot reflexology, Indian head) = €30
50 min massage (as above) = €50
Shiatsu massage also available but think it costs a little bit more...
Manicure = €30
They also offer the usual other beauty treatments, facials and body treatments, I don't know the prices off the top of my head but you can make a rough guess based on the above. You can also e-mail the hotel on:
booking@hotelprincesaparc.com maybe they will send you a price list if you ask.
Also worth knowing that flip flops are obligatory in the Wellness Centre area so you can pack appropriately!
Hope that helps,
Emma