Some traders, who accept cards, go for numbers, some for value. 30 cards were 1 TF2 key, but I haven't done that trace since 1 month. You can see the price of a cards from steam card exchange. For those who accept in numbers, I think 5-10 for a bundled game is okay.

I can't really say how cards translate to keys but from my experience many cards that sell for 5-7 cents if you want them right now can be purchased for 3 or 4 if you are not in a hurry. So if you don't really want the cards but just accept them I wouldn't price them any higher. I normally bid the lowest price they have sold in the past 1 or 2 month (unless that was just a single copy)

just set minimum price of card you accept and just give amount based on that price?
for example, set that you don't trade for cards that cost, lets say, 7 cents. so you get 5 cents netto back. take that 180 and divide to your 5, getting around 38-40 cards. and set that number, in here, for 40 cards. and don't worry about what you get while all card cost 7 cents or up.

or deal in those gem packs, those things don't have week trade / sell "ban" :)

I find cards too differently priced for using them as currency ... I've seen 5c ones and also 15c+ cards (of course the ones I want the badge are always expensive ...) :(

Usually 5c-6c each... people are always gonna give the lowest valued cards... that's why some traders sometimes do (15 cards of your choice or 12 of my choice)

After doing some trades in cards I notice that most traders try to select the cheapest cards possible.
Hence making the average value of a card around 3,5 cents.

I usually only buy bundle games with cards and 5-10 is pretty standard unless it was a BTA or one of those in HB's $15 tiers. Most of the people I've traded with that were willing to take cards instead of keys had a 40 card exchange rate.

If someone offers you cards, then i value them less than their net worth on the market, because i have to invest time and sometimes luck into getting money out of it, plus you have to pay additional fees, if you want to buy something from that money. Also, the users get cards for free, for only playing games, so they are not really worth their price on the market.

If someone has enough cards, he can sell them himself and buy me a key or item i want. That's how i do it.