I had a lot of positive response for Bribe, Craft, Train and the related posts on building resources like arcane libraries and holy shrines. However, at least one person asked for a table, since that would make the whole thing easier to understand. I thought it over, tried a couple different layouts, and settled on one that I thought would make the most sense: the Investment Cost Table.
The table assumes that the player tells you what they want to do and how much they spend. The player does not know the difficulty level, usually, nor the exact chance of success. You would look up the difficulty level in the header row, then read down that column to find the amount spent, or as close to it as possible without going over. The first number in that row is the target number to roll under with a d20.
The table leaves out some values: 6 to 9, 11 to 14, and 16 to 19. These are all 5, 10, or 15 + another number, so I figured this would be more compact and still easy to do.