Very little - probably order €0k-€10k.

Entering slavery is an act of desperation, taken only by people who are trading everything for a warm bed and regular meals. They have no ability to argue for higher recompense since their choices are so dire.

It's possible a small boutique market in salaried "slaves" would emerge, similar to the market for footballers for particularly high performing slaves where there are big money trades but the money here would gogoes to the owner not the slave, similar to how the transfer market for footballers works in the real world.

These slaves would end up being well rewarded since there is no other way to make them work well. Since slaves cannot own property, this would have to be in the form of benefits such as luxury houses, access to private chefs, high end clothes and cars, etc.