It's been awhile since I've created a custom lord, but I remember the costs of things getting progressively higher the more that you buy. Do you already have any units/spells/artifacts in your inventory? Try deleting them and prices should go down. Another trick you can use is to purchase an expensive artifact, but don't drag it to your Lord's hand. Instead drag it to the bar at the top where your artifacts are stored. Now, if you sell it, there will be an invisible "ghost" artifact in the same spot, which you can also sell for the same price. You effectively get double your money back.

That was it, I wasn't scrolling down to see the other factions. Thanks for the help!