With Custom Pricing Rules, you can use logic to modify the total price when specific conditions are met. You can also include a processing fee (fixed-price and/or percentage) to add on to the total. They're great for any pricing structure that's a bit too complex to be handled using Product and Price fields alone.
If you want to achieve some seriously powerful dynamic pricing, you can do just about anything using Custom Pricing Rules with a Calculation field (see below for more info).
To configure Custom Pricing rules:
To add the answer to number questions or calculation results directly to the price, you can do with the following setup. This means that when the targeted question is answered, we're simply going to add its answer to the total price. You could also subtract, divide or multiply the total price by that answer.
You can also use a calculation's answer to override any other pricing on the form, replacing totals from Price and Product fields rather than adding to them. To do this, select "= answer" instead of "+ answer" in the last dropdown pictured above.
If you're using Coupons on the same form, it's important to note how these will interact with your Custom Pricing Rules.
To put it simply: coupons with set amounts get along with CPRs that add or subtract from the total, and coupons that give percentage discounts get along with CPRs that multiply or divide. Other combinations might not always work as expected, so it's a good idea to test out different combinations on your form to make sure the total is always correct.