I don't have a government statistical source for you but you should be able to use your common sense to accept the validity of the assumption. If you needed to take a package across the city or say to Leduc. Would you use a bicycle or a vehicle? Presumable you'd use a vehicle because you should intuitively know that's it's the more efficient way of doing it. Therefore the GDP generated by using a vehicle is greater than by using a bicycle in virtually all instances other than short distance. And does some leisurely vehicular travel bring down the size of the parameter? Sure, but cyclists use roads too.
What is the source of your public benefit assumption? You're simple substituting the free parking benefit from a homeowner to a cyclist. You can't compare an individual homeowner to the aggregate of cyclists. You need to compare the aggregate number of homeowners to the aggregate number of cyclists if you want to assign value to the free parking. And if wish to assign value to the free parking than you also need to assign value to the cycling path and the benefit cyclists will receive.
Ah... the "common sense fallacy"... I'll flip your argument back: if you need to drop off a package from, say, Winketowin to Whyte ave, which, using "common sense" is probably more frequent, considering that these are two of the densest populated areas of the city, and you use a bike, the net GDP generation is higher than by private vehicle, since all costs are lower: purchasing, operating and maintaining a bike, building and maintaining the infrastructure, not to mention the lower environmental impact and the health benefits to the cyclist, which in turn adds economic benefit for the society as a whole...
And the public benefit assumption goes beyond just the cyclist himself: it has been proven, with actual evidence, I might add, that having dedicated cycling and pedestrian infrastructure lower the risks of accidents, and the severity of the ones that still happen, which is an overall public benefit, just for starters. Also, while the bike lane is a public thoroughfare that can serve several hundred people, having street parking, especially in residential neighborhoods, provides no public good except that to the homeowner who, I'll say again, should be the only person responsible for the costs of having a private, dedicated parking spot at his front door.
Not to mention that you keep treating bike lanes as this leisurely and fringe thing that only serves a radical elite, but ignore the fact that the underusage of bikes as a daily transportation method is also a reflection of how the lack of proper infrastructure makes riding them unsafe in most of the city, therefore forcing people into other modes (usually motorized vehicles). Imagine if the script was flipped, and instead of having 11000km of roads and 100km of bike lanes (and these are not contiguous), we had the exact opposite, now imagine that all of the space dedicated to bikes in this scenario are very narrow lanes that fall into a cliff, making it relatively safe for cyclists, but extremely dangerous and hard to navigate by car, and accidents would often be fatal do drivers. Would people still choose car ownership, or would they start riding bikes more often, and only those with dedicated car lanes would frequently use their cars (while still owning a bike, because they will often need to go to areas without car lanes). This exact what happens to cyclists who want to use bikes as their main mode of transportation. Riding on stroads designed for high speeds, negotiating the road with drivers in their 2-ton suburbitanks who see them as an inconvenience, or breaking the law and sharing the road with pedestrians (and risking getting penalized for injuring one accidentally).
