[militarist]

It really depends. I believe 2-4 slaves is reasonable, but it really depands on competition. Once I played hinnom, and death was closed for me during the long period of time. I had awake fountain of blood as a petender, so had a lot of slaves to some moment, and have bought a lot (I think around 100) of D gems from Lanka in exchange of slaves. The price I payed was 6 slaves for each gem. It's really expencive, but there were several reasons:
1. Death gems are one of the most valuable gems in a game.
2. I was leading all charts, and nobody wanted to trade me death gems. Lanka was the only proposal on a market.
3. I had many territories, so I expected very good D income which will cover expences anyway.
4. I was researching towards Cons 8, was going to be there first and needed D gems to buy artifacts first (and some D artifacts really shine)
5. I didn't have any wars, so didn't have immidiate usage for slaves anyway.
6. Lanka wanted my slaves..but why would Lanka pay a lot for slaves, which it generate perfectly itself?
7. It's Lanka proposed this price and just refused to negotiate, and I had no choice at all.

So, a combination of factors led price of D gems for me to 6:1. And both sides were happy. So, the answer - there are some numbers you can start bargain from - if someone contacted you for exchange, but the final deal depends only from how badly you need what yuo need, and do you have a choice between trading partners.