$90k annual gross sales, you might be able to make 35k wage. But you can make more just working as designer at $17/hr, 40 hr week without risk.
Business value generally breaks down to: FF&E, inventory, trade name, goodwill.
Furniture, fixture , equipment & inventory are fix assets which should be listed & caculated accurately. Trade name & goodwill are intangible assets that are valued according to historic sales & profitabily of the shop.
$10k/mo is about break even figure after paroll. As certain overheads are fixed, the more sales the higher net profit margin. If you can reach $40k/mo. You can net $4-6k/mo. plus salary. That is pretty good.
When evaluating a business, look at the up-side protential. You can going in cheap, build up the business and accumulate asset. Or pay a premium for a successful business, let the profit pay for your investment.