Well...... a select few top pro riders make a lot of money. Most of them make fairly regular to meager salaries... (and even the top salaries are peanuts compared to most stars in other sports - even considering all of the shilling for their sponsors cyclists are expected to do!)
That said - yes the money in cycling is all based around advertising. Everthing has a sponsor - from the timing clocks to the sprint points. While there is no admission to watch a race many people do come and advertisers take full advantage of those waiting for the race to come through. We don't generally see any of it on TV, but before the racers ever come close there is a huge advertising caravan with floats and crazy cars shaped like products of all sorts and people throwing out samples to the crowds. Of course there's always merchandise sales as well - there are tons of T-shirts, caps, replica yellow jerseys,polka dot socks, coasters, beer glasses, water bottles....