pricing for custom units

i have a very good understanding of most aspects of custom units with the exception of one of the most important: pricing it. does anyone have a system for that?

Take the cost of a unit it is similar to, work out costs for upgrades based on existing ones, and go from there.
This is flawed when I created a 6-model 'Coven' for the Poisoned Tongue and it came to over 300 points. When that sort of thing happens, common sense dictates.

thansk. how do you price increases in stats, though?

That's a much harder one, as it depends on the stats of the unit you're upgrading. For example, T3-T4 isn't so big, but T4-T5 is, so those two jumps will have different costs. Usually from 3-4 is 5 points ofr S & T, 4-5 is 15-20 points and so on.
WS, BS and I are cheaper than S & T for obvious reasons, except WS8-9. That's a big jump.
Pretty much, work out how much more powerful the jump makes the unit and go from there.