I'd like to know, to which degree does the Gas prices from the main eth network are comparable to the ropsten test network.
I don't think you can compare those two. The mechanisms are completely different.
In mainnet it's a matter of optimizing mining revenues. This depends on factors such as congestion, block gas limit and possibly Ether price.
In a test network, such as Ropsten, nobody gains any monetary reward for running a mining node. I'd imagine people run nodes mostly for fun, to learn the technologies, to test out new features and so on.
Even if the gas prices would happen to be similar, you shouldn't count on it.