Hi Alice Star,
Your repayment amount will depend upon the source you borrow from. If you have friends or family (typically called 'love money'), there may be no interest and an open term to repay it - but, the caution here is that many people take a little longer to repay friends or family than they ought to, and that can cause hard feelings to develop, and those are difficult to heal.
As for a method of calculation, $600 at 10%, for example, would cost $60/yr, but that is 'calculated monthly, and added to the outstanding balance.Here is what happens with the first payment. Let's say your payment was supposed to be $50. First, the annual interest is calculated, then divided by 12 = $5. So, your balance at the end of the first month is $605, then you deduct your payment, giving you $555 balance remaining.
At the end of the second month, $55.50/12 interest is added on, making your balance $555 + $4.63 and your second payment is applied, which will give you a remaining balance at the end of the second month, of: $559.63 - $50 = $509.63.....and so on.
If you could manage $100 per month, the balance at the end of the 1st month would become $505, after interest and your payment, then at the end of the 2nd month, you would add on $4.21 interest, giving you; $505 + $4.21 - 100 = $409.21 remaining, compared to $505 if yo only pay $50 per month. You can see the advantage easily.
I once had a small department store balance (28.8% per annum). I worked out how long it would take to repay it, if I paid the minimum $10 payment monthly, on a balance of about $50. Doesn't sound like very much, does it - and, one probably figures about 4 years, but when interest is added on, I discovered it would take 7 years and 2 months = $860.00 over that time, to repay $450. That is called compounding (as I showed in the paragraphs above - the same method), and yes, it is possible to nearly double the amount you eventually wind up paying back, by only making minimum payments.
Bottom line is, never borrow (of course that is impossible for me and most people), and pay back quickly (if possible). But, you see how it works.
Banks might give you a credit card, in which case you just follow the minimum payment rules if times are tough, and be sure not to miss a due date. That way, you can repay the entire balance once things pick up, while creating an excellent credit rating in the meantime. Their rates (at least here in Canada - tend to be about 18 to 20% per annum (1.5% to 1.67% mo - which isn't too bad).
If you know your bank manager quite well, you could ask if there are alternatives to credit cards, perhaps a small line of credit which doesn't have fixed payment amounts, but just lets you pay the interest on the outstanding balance, and I think that most banks got out of the business of demand loans many years ago.
Payday loans seem to charge about 30%, but the only problem with those is their allure. They are so easy to get, and so difficult to abandon, so your $600.00 debt balloons over time, with such an exorbitant interest rate. And, it is futile to criticize their charges; they charge that much because governments let them.
The only other choice I can think of for raising cash fast is selling things (but you have to be very careful of the creeps who want to come see you stuff), either advertising (which of course costs money), or pawnshops, but, at a pawnshop, you will be very lucky to get 10 cents on the dollar, and their manners are terrible - kind of a 'take it or leave it lady/buddy approach, which can be really hard to endure. And, if you should decide to pick something up once times are better, you will pay one whacking great interest rate to them - not sure what it is, and, I think you only have 30 days before they put the item up for sale. Not my first choice!
I hope this gives you some help and hope.