How much should I pay in refined for the portal bundle while it's on sale? (3.74$)

A little over key, I would guess. But a lot of traders are greedy so they'll try and get you to pay the full price. (Don't remember how much a key is, haven't played in a while.)

10ref or so would be quite fair. Having in mind that its actual money involved on the other side. You migh be able to get it for less though. A ref is roughly 40cents

refined is around 0.50$ or so? correct me if i'm wrong.

So i guess 8 is a good way to go.

One does not simply leave TF2 forever. Back to topic as others said find around 3 keys don't go and overpay :)

2-3 keys. I suggest you do what loveyourawr said though. Sell them in the market, that way the chances of being scammed are 0.

keys sell for $1.60-$2.00 on steam market each key is 3.11-3.33 ref