Location, location, location. In my area they have shirts for $1 on Fridays at Salvation Army. Independent thrift shops run by churches & other groups sometimes have them for less. I know that in other areas prices are much higher. I would ask friends to see if they have any to contribute, or as previously mentioned, wait till the summer and check out garage sales.
Also, remember that a long sleeve men's shirt can yield well over a yard (meter?) of usable fabric. So for $6-10 that's not a bad price to pay.
I've made half dozen or so recycled shirt quilts. The think I really like is that the fabric has been laundered, so it won't shrink or bleed.