It's always difficult to say without knowing how well priced the work is and how good the access is. For me, a list of work where having to text in advance, has a much lower value than a list of "turn up and clean". For others, that may be less of an issue.
Twice before I've bought work, and both times I felt either very disappointed or mildly disappointed - but I'm probably fussier about what I take on than most.
I suppose if the work is reasonably priced with reasonable customers and only a smattering of pre-notifying, it might be worth anything from 2x monthly value to 5x.
If you are offering to accompany on the buyer's first cleans to supply a personal intro, maybe you could squeeze more out of it. It also depends how well established it is because some will likely drop out in the first few cleans.
So, it's really hard to say. Much depends how you dress it up and what, if any, guarantees you are prepared to offer.
A buyer will likely be aware that if someone is only selling part of his round, he's unlikely to be selling his best work.
EDIT: The first time I bought some work, it ended up at about 4.5 times monthly. This was paid up front but I did get a personal intro. With exceptions, the pricing was decent. There were access issues at many properties though and in the longer term, I let go of much of the work as some of them needed to be in for me to clean. Some cancelled. Two of them ripped me off for the money. I ended up keeping the best 5 out of 23 jobs in the longer term. They are decent jobs though. Knowing what I now know, the £2k I paid would only be about £500. The second time I bought work cost me much less but generally is priced lower. Access is mostly OK. For this work, I worked it twice with the seller getting the money, and now it's mine. A couple dropped out and I'm likely to drop a small amount too. Although it's mostly OKish for pricing, several need a big increase. I anticipate losing them. In all, it started at around £600 per 6-weekly clean but will probably be around £400 by the time the dust settles. Those figures aren't exact but it works out at around 2.7x monthly clean and I didn't have to lay out any money.