Lets say the players want to hire someone for a skill check. A slicer, Knowledge, Thief to pick a lock

How much would it cost?

Now granted sometimes it would not have a cost measured in credits. But would you have a ball park figure for credits based on the skill?

The better question is, if they pay money for this NPC's assistance, will the GM still roll for the NPC, or will he or she play it as an automatic success.

Ask yourself how much Obligation they'd incur if they asked for the assist in that way. If you're dealing with low-level stuff, then maybe 2-3 points. If it's high skilled work, then 5 or even more. But assume 3-5 for the moment. For starting Obligation, you can get 1000cr for 5pts. In an example in the GM section of the Core Book (p.310) there's a suggested 5000cr per point. A range of 200-5000cr per point is pretty large but gives us a range to work with. So...

Easy - 200cr

Average - 500cr (common skill, legal checks) to 1500cr (rare skill, illegal checks)

Hard - 1000cr (common skill, legal checks) to 5000cr (rare skill, illegal checks)

Daunting - 3000cr (legal checks) to 10000cr (illegal or dangerous checks)

As kaosoe mentioned, it matters whether it's an automatic success or not. My little chart above, assumes it's an automatic success (but roll for time spent and threat/advantage). But the PCs could pay the category below for a possibility of success instead. For example: Pay 500cr for someone to slice into a restaurant's back door. If they paid 1000-1500cr, the Slicer would be good enough to do it automatically. But at 500cr, you've got a talented kid from the neighborhood who will probably succeed.

There is a really nice chapter at the end of dangerous covenants that has a table that shows the average credit rate for the average mook job. That's a good place to start looking and a good book to have in general for gm's.