1. How eager you are to gain the work
2. How busy you are with other work, i.e is this work expendable?
3. How much you think the client is willing to pay
4. How much you think the job is worth / how much time involved
5. How much you think other developers would charge

Unfortunately there are no hard and fast rules and I wouldn’t listen to an established freelance developer if you are just starting out and want to gain the work.

Figure out an appropriate hourly rate, guestimate your hours, and calculate your project costing on that basis. Freelancers will often add a small 10-15% increase to act as a buffer for all the stuff that was not anticipated, e.g. testing, bug fixing, tweaking, last minute client amends etc.

Then take this final figure and run it by the above questions to check that it seems OK on each front. When you’re happy, pass this value onto the client.

You can charge what the market pays.. and I suggest doing an hourly scale as you have no idea how much time porting the existing theme is going to take.. along with the products