TL;DR
"How much should I charge?" would be off-topic. Calculating costs, especially retroactively, should be a straightforward formula based on your contract with the client.
Calculating Costs
There are generally three kinds of rates:
- Fixed rates, where you and the client have agreed on a scope and a price ahead of time.
- Time and materials, in which a fixed labor rate is multiplied by the man-hours consumed by the project. The cost of externalities such as equipment, consumables, or travel expenses are generally billed as separate line items from the labor.
- Receipts made, in which you pass your actual costs onto the customer directly, plus any agreed-upon discount or mark-up.
If your contract specifies a fixed rate, then you can't change the price; your primary project controls are scope and volume, which you can't really manage retroactively. True fixed-rate contracts shouldn't involve any calculations, since the rate has already been agreed-upon ahead of time. However, if your "fixed rate" is based on some kind of piece-work, then you would calculate UnitRate x UnitsCompleted.
If your contract is for time and materials, then you normally calculate (LaborRate x Hours) + Expenses to arrive at the cost. If your contract is on a receipts-made basis, then you typically calculate MoneySpent + NegotiatedMarkup.
Read Your Contract
In your specific case, you've already done the work. Presumably, you didn't do the work before negotiating an agreement with the client, and you should therefore have a written agreement or signed contract that spells out how you may charge for the work performed and how those charges should be calculated.
While there are some standard ways to charge for projects, a contract can specify any billing structure you and the client agree upon. As a result, you need to refer to your contract to determine how costs for the project should be calculated and billed.
