Well, how realistic vs good game do you want to play it?
Part One - Repo Men:
In Reality:
Repo Men (aka Skip Tracers) generally make a fair living but nothing spectacular, and only after an apprenticeship. Its a volume business, the more repo's you can complete the better your income.
You're not likely to be given a "golden repo" unless you are yourself a very well known agent and you are working in an environment where such an item comes up. Most really high price items will not have to resort to repossession, taking other routes to fix the credit.
Now, that goes contrary to the canon on Starship loan defaults and skips and the implied network of Skip Tracers so...
In Gaming:
I'd suggest a few small repo assignments on behalf of some mega-corp bank that holds loans on everything from starships to scooters and start the PC's small. First give them a few assignments all on one planet and all for small items. A ground vehicle or two, maybe a mobile home to start. And then a bigger vehicle (maybe a grav or small craft) to wrap up the apprenticeship. They should be losing money, i.e. earning far less than basic living expenses for 2 people, on the first few and maybe breaking even on the last one. If there are more than 2 in your group then they are making even less.
Don't forget that maybe the owners will notice and get involved in the attempt with sob stories of "we sent the payment last week" and such which are almost always lies but may be effective on the PC's. Only rarely will the defaulter respond with violence.
So how much is fair? Probably no more than 10% of the monthly payment on the vehicle, and that's for the whole deal, to be divided between the repo crew, after expenses. Less reputable repo outfits may add other "fees" such as storage and transport (if the "owner" makes good on the outstanding payments) and personal property storage (items found in the recovered property). These fees of course are charged to the "owner" not the lender and its up to the repo firm to actually collect on them

Such fees will usually be in the same 10% of the monthly payment for the storage, per month, and some flat rate on the transport (Cr50) and personal property fees (Cr25 per month).
Now once the PC's have proven themselves capable (if they do in fact) then they may be given a Starship skip to trace. That should keep them busy for some time (months) and net them a good profit at the end of it* but nothing before then. No expense account and no retainer, not until they have a few good recoveries to prove their value.
* IF they keep their expenses down. If on the other hand they are trying to make their own payments on a ship at the same time by trading and such they won't have much "free" time for tracking leads and someone else will be given the assignment.
Also, Starship repos are going to be a bit dicier. First the ship is often armed, as are the crews, and said crews are usually a little more prone to violence, especially since they may be facing prison time for the "theft". But that gets into the bounty hunter side of it...
Part Two - Bounty Hunters:
Here's a website that should provide some insight:
Bail Bond Enforcement