In order to calculate the amount to pay inside a Blippa Form, you first need to enable at least one payment integration (Stripe, Swish etc). (In Labelsadmin-Settings-Integrations).
- Optional: Add a system variable with the price. Make the variable Numeric and do not select Account object etc. (This way you can later change the price through a form instead of having to go to every payment form.)
In the form, add a dropdown and list the choices.
In the form, goto Setttings:
- Select the payment option.
- Add the price, either in the “Amount to pay” (for single price) or “Calculate Amount based on Form field”.
In this case, the variable stallplats_dygn is the daily price for one RV.
And for two days I just multiply the price 
But you can do much more complicated calculations, like
($var:hamnpris_dag$ + ($var:landstrom$ if '$form:extra_landstrom$'=='Ja (100:-)' else 0))*(0.9 if '$form:discount_svsjo$'=='Yes' else 1)
The result:
Have fun!