I am not a big fan of kits - but have bought a few.
It really depends on the quilt shop who is packaging it. The few kits I bought were sufficient in the fabric yardage to do the exact quilt, nothing more. One kit I purchased from a huge shop was a kit for a Holiday plaid star wallhanging. There were extra FQs in the package - very nice in that you could better choose your colors. The downside to that kit was all the FQs were not cut on the straight of the goods so there was lots of wastage as I cut square. Another kit I purchased on clearance at Connecting threads - that one was ample enough as far as yardage - I had quite a bit left over - but again - not enough to have changed the pattern drastically.
Hope this helps. I have found - if you are thinking about changing something - then it is easier to purchase the pattern and pick your own fabrics.