I mostly buy kits that I know I will get more than one use from. I rarely mix and match kits (not clever enough) and I think I have made about 13 LOs from this VC by Lauren
Bennie Value Collection
I have used this set by Lauren numerous times as well
Quite Bright Value Collection
and I have used the backgrounds from this set by Suzanne C Walker more times than I remember
Just Plain BackPack
Just Plain White Page Kit
I have numerous other kits that I really love, but these are the ones that I have had the most use from.
Oh yes and to actually answer the questions
, my DH and credit card think that I have spent way too much on kits over the years
I rarely buy a kit specifically to do a challenge unless I really really like it, and if I really really like it and can see that I will have multi use of it, I will pay top dollar.