Commerce : products with 2 prices
Pricing rules was the right track. In order to configure 2 main prices for a single product :
Add a 2nd price field to the product type:

Add a boolean field to the corresponding line item type:

In the Add to Cart form settings of this field (admin/commerce/config/line-items/CUSTOM-line-item/fields/field_avec_aerien), check  Include this field on Add to Cart forms for line items of this type.
Finally, in admin/commerce/config/product-pricing/rules/manage/, create (or clone &amp; edit) a new pricing rule which set the unit price to 2nd price amount, when the boolean is checked by the customer in the add to cart form.

Pricing rules are very powerful, indeed !