I recall running a few months where I sent out 50,000 and it cost me about $1600 for the 5 zone package, it paid for it's self by 3:1 but wasn't a killer return or anything, just kept the crews busy as I was focused on cross selling window cleaning and power washing to carpet custies.
Pray you don't have a window cleaner marketing to your base, cause he'll be in there 4x more often than you will...lol
It was also a good defense, once established you Had better be pretty diversified to crack that nut, if a window cleaner happen to solicit, Mrs. Piffelton would say sure I'll accept a bid, but you have to do my carpets and furniture too, that kept many out.