We just had our interlock patio done by a professional contractor this week. If you are interested, I can give you a few names and references so you can get estimates.
We ended up going with FM Interlock as they gave us the best price, also longest warranty (3 yrs) compared to the general 2 years. But every contractor quotes differently depending on your grading, area, etc. To give you a ballpark figure, it costs around $20/sq. ft. with medium grade pavers and steps.
