Pisac is rather small and ,with the exception of a few historical attractions, there is not a lot to see. It's best to go to Pisac on TUE, THU, or SUN which are the days that the market is open. Take your time and browse at the market. Each stand carries similar items but sometimes it takes a little patience to find the right color, the right pattern, the right size.
What to buy: Scarves, gloves, hats.
What to pay: My advice is H A G G L E! I'll be the first to admit that I need to work on my bargaining skills but why pay 20 soles when a little haggeling will bring the price down at least 3 - 5 soles!
In sunday market there are a lot of stalls selling souvenirs, and wool items made of alpaca generally: pullovers, scarfs, gloves... They are basically the same price as in Cuzco, but you might find some more variety here, and you can even bargain a little more here than in Cuzco stores.