percentage in products

Can i somehow have a percentage on the product, what i mean is…

Lets say i choose from the first step a product i want…can somehow the produst of the second step cost +30% of the first step?

For example i chose from the first step the product A that costs 10$ and when i go to step 2 the poducts will cost 30% more, 3$….but if i am at the first step and i choose the product B that costs 20$, on the second step products will cost 6$.

i am willing to pay you if i have to so that i can have that…

Thank you.