My answer to this isn't entirely about supply costs. Yes, starting out you may need to invest in a lot of supplies that will take a while to earn enough to cover the cost. I think you should start small and not put supplies on credit cards when starting out, if your carrying those supplies as a balance they are costing you a lot more with interest. How much is totally dependent on how much time you can devote to your craft, what supplies are needed, and how much you can afford.
The real issue here sounds more like covering the cost of your supplies on an ongoing basis. If you are selling your work yet can't cover the cost of new supplies to restock then your prices are probably too low. You should be making enough to cover the cost of material, the cost of a replacement, and pay yourself a decent wage for your time.. I've covered pricing before in this post. If on the other hand you are pricing well and aren't selling enough to cover your costs then you may need to take a break from making new pieces and work on promoting yourself. You need to be selling enough to cover your costs and make a profit.
Make sure you are tracking all your sales and expenses somewhere, whether it's accounting software like Quickbooks, Excel/Open Office spreadsheets, or a ledger book. Doesn't matter so long as your keeping records. If you aren't tracking this stuff it can be far too easy to spend too much on supplies or spend the money elsewhere. Plus if you keep the books organized it makes taxes far easier and you and the IRS will be happier.