The reason Disney Interactive is calling the first 20 Power Discs "Series 1" is because more are planned. The official Disney Infinity website has teaser images in place for Series 2 and Series 3. If we assume that each of these new series will be identical to the first then you're looking at 60 Power Discs total, 9 of the "rare" variety.
The Series 2 teased Power Disc looks like A Nightmare Before Christmas landscape. The Series 3 Power Disc appears to be the Alice in Wonderland flamingo croquet mallet.
Each two-pack of Disney Infinity Power Discs is priced at $5. Assuming you are lucky enough to not get a duplicate, it will cost you $50 to collect all 20 Series 1 Power Discs. A more accurate cost if not buying from the secondary market or trading is probably north of $100 with a stack of duplicates to deal with.
Families that do get bogged down until a stack of Power Discs might want to consider coming up with other creative uses for them. Since the Power Discs loosely resemble poker chips, it might be fun to develop some board/card games where the duplicates serve a purpose.
You can get a head start on building your Disney Infinity Power Disc collection by pre-order packs at Amazon.com for $4.96 each. Buying six at once will earn you free shipping.