...as opposed to buying it?

What would a good figure be?

Making what?

To make anything, you've gotta have the appropriate tools AND the materials necessary. If you wanted to build a gun, the cost of tools + materials would far exceed the cost of just buying the gun. If you wanted to build 1,000 guns, it may be cheaper to buy the tools and materials. If the runners value their time at all, they're better off buying.

Most of the time we consider that the materials cost half the price of selling the money (with price fluctuations depending on availability of material, etc...). The tools are another part of the problem. Some things require a better workplace than just a tool kit. For instance, you can build a gun or rifle with a "weaponsmith" tool kit, but you can't build a car with just a tool kit, you need at least a tool shop.

Usually these issues with "how much to charge the runners for making something" comes up when some player gets the bright idea to make money building and selling some item, usually contraband. At that point, you tell them that Shadowrun isn't an entrepreneurial simulator but a roleplaying game, mostly focusing on combat and infiltration.

If it's a runner looking to make a special item for themselves or the group, make sure the player has the appropriate B/R skill, the appropriate equipment, and then I usually go with 50% of the cost of an equivalent item to what they want to make as the materials cost.