Can I create variable pricing on my forms?

for example, can I charge different amount based on choices in a form? Can I set a maximum price for a form?

Yes, the best way to do so is to assign your options calculation values:

https://www.jotform.com/help/301-How-to-Assign-Calculation-Value

You can also use conditions to update values in text/number fields:

https://www.jotform.com/help/268-How-to-Insert-Text-or-Mathematical-Calculation-into-a-Field-Using-a-Condition

Then you can perform further calculations on those values using a calculation field:

https://www.jotform.com/help/259-How-to-perform-calculation-in-the-form

Once you have your total, that can be passed to a payment field for payment processing:

https://www.jotform.com/help/275-How-to-pass-a-calculation-to-a-payment-field