How do you value collections when you buy them?

Just curious. I am not looking to buy large collections at the moment, but I'm wondering how a person puts a value on the collection (especially very large collections).
Do you consider the value of every pin, even though it could be hundreds or even thousands?
Do you have "rule of thumb" value for different types of pins, such as LE, OE, HM, etc.?
How do you do it?
Thanks in advance!

Depends on the collection.. I've bought & sold several and each had its own rules depending on size & pins.. I also was buying/selling for good friends so while I had to 'get something out of it' I was more generous than if I bought a strangers collection..

generally 'junk traders/mickeys/booster packs' were worth $1-1.50. OE/not popular LEs $2-3, after that I generally paid half of the lower end of estimated worth (if a pin was selling for 20-30 on eBay I'd pay $10) then we'd split the eBay selling fees in half-

Thanks, -em. That makes sense. Looks like it may take a long time to evaluate a large collection, but I see that it's important to do so.