If you need to purchase something for the project I generally sell it to the client at cost and work the cost of the purchase into the overall price. Most clients would expect you to add a markup to purchases but they're pleasantly surprised when you don't. You still get your money and you get a happier client in the end. Everybody wins.
So if contracting, enough to cover your cost of life, either from the outset or taking into account some forecast of growth due to experience or ability to scale. The more forecasting involved, the more frequently you need to evaluate it.
Sorry, I know not that helpful - but I do mean it with sincerity. It forces you to think of the fundamentals and avoid expensive wrong turns.
In theory you should try to charge as much as your clients are willing to pay for your service. That might be very different amount based on how valuable the service you provide is to them.
Charge more until you stop being flooded with work requests.
