Lyft with the funding from GM has only raised $2B over it's lifespan which is comparable to that of Uber. Uber has raised nearly $13B and blown through most of it.
At their last round, Lyft is valued at about $6B and Uber is valued at $68B. One of these is reasonable based on the taxi market size (domestic $20B), the other is about 2x over the assumed value with decent market share and margins it could attain.
The only way for Uber to express the value on the books is to replace car ownership, which is unlikely with the turnover rate of individuals replacing their cars, they'll need about a decade or more runway to justify the current valuation.
Lyft and GM have an advantage in the race to replace ownership, with their partnership. Lyft gets a partner that can reduce cost of vehicle supply costs and maintenance, while GM gets a hedge against a possible headwind to their current business model.