My memory in general isn't the best, but it's eidetic when it comes to Valve and Saoirse-related stuff.
Except as I posted that isn't what Valve does to buy maps, the map stamps and strange filters are just a way for TF2 players to donate money directly to community map makers. I'm pretty sure Valve added those to the store because people making weapons and hats were making a lot more money then map makers and Valve wanted to let people give some extra money to map makers.
For example there is an interview with Robin Walker where he explains the process. The site the interview was on is down now, but someone on a Gamefaqs post copy and pasted it a long time ago:
eXtine: There was some discussion about potentially compensating L4d mapmakers whose custom campaigns are of high enough quality to be released as DLC for Xbox Live. What is the relation between Valve and community mapmakers in regards to TF2 and what happens behind the scenes when a map becomes official?
Robin: Right now we simply purchase the map & the associated sources from the folks behind it. It's pretty straightforward.
Also here on the TF2maps.net forum the creator of 2 community maps added in the July update talks a bit about the process:
If Valve knew what they were looking to purchase from mappers, then we wouldn't really be surprised or excited by anything that a mapper made.
He confirms in that post that Valve purchases maps.
Below he lists the process that Valve goes through when buying maps:
P.S. For those wondering, here is a summary of the emails between me and Valve. None of this is word for word, just a quick boiling down of how the map went from my hard drive to Valve.
V: Yo, we would like to acquire process and standin.
Me: Cool! Here is a list of everyone involved!
V: Is this your current address?
Me: Yes.
V: Okay, here is a contract, please sign and scan back a copy to me. Also, we will need any vmf's and textures associated with the map.
Me: Hey, do you have a timeline? I would like to make some changes to the maps before they are put in the game. Would that be okay?
V: We have already playtested the maps as they are currently publicly available, if the changes are too extreme we might have to retest, and we may not want to purchase them.
Me: Okay, I'm only planning slight visual changes. (send the updated versions on sunday)
V: Hey can you fill out this form so we can wire you the money?
Me: YES!!! I CAN DO THAT!!
Valve has no problems buying stuff created with their own technology if they want to (in this case it's their map editor Hammer). Valve has even hired people to make maps for them, the map Kong King released as part of the Sleeping Dogs promo last year was an original community map they commissioned.
Most sources I've seen online says that Valve pays several thousand dollars for maps. I imagine it varies per map, since if they have to buy a lot of art assets for a map that probably all comes out of the same money.