The variations table on product edit screen contains a small drop-down:
Three price modification types are available:
- To add a variation price to total use the + option
- If a value in the price mod column is a percentage use the % option – it will add a percentage of the main product price
- If you want to set the main product price to zero or leave it empty use the = option. It will set a fixed price for the variation. If you set multiple variations as ‘=’ the final price will be a sum of all the options.