Your bill is priced that way so that you can have predictable price for what you consume. That means sometimes markets electricity at zero, but you'll pay the full contract price. Other times, electricity is _much_ more expensive than your contract, and, likewise, you'll pay a fixed price.
That fixed price is not competitive with balcony solar because it pays for services that balcony solar doesn't provide. That means that on average, your solar panels produce electricity when electricity is cheap for your utility provider, and when they don't, electricity is on average (but not always) more expensive.
The logical consequence of this is that your contract is going to become more expensive, because you use your solar when it would be profitable for your utility, and you draw from the grid when it's unprofitable to them.
Another alternative would be the end of fixed cost contracts, where balcony solar users would have to pay market price + subscription for their electricity. If that was the case, balcony solar users would probably spare cents during sunny weekends, and get hammered during winter workdays when there is no wind.
edit: just checked the data, in August, Germany's prices at noon hit 0€ 17 days. On the other hand, prices at 18h soared above 100€/MWh every single day, and above 150€ 10 days in the month. In Sepetember, prices at noon hit zero 14 days.
Another (wind-related this time) example, market electricity prices this weekend were negative most of the time (with a tiny 4€ peak) for 40h straight.