I believe most EU countries require you to pay the social/health insurance on a "virtual salary" (usually tied to minimal wage or similar.) This is to curb evasion of the social security payments by setting up "businesses" whose sole purpose is to provide labour to a single employer.
In the Nordic countries, at least, that only applies under pretty specific circumstances (which try to target the tax-evading cases you mentioned). In a normal case, if you're self-employed, you pay social contributions only on what you actually make, which might be considerably less than minimum wage if your business is not very successful. Actually have a friend who recently filed taxes in that circumstance, since his one-man mobile-game studio didn't sell very many games in 2012.
