Every contract requires there to be "consideration", which means each party is obliged to offer something valuable in exchange for whatever valuable thing the other party is to provide. In the simplest case, that would be money in exchange for labor. Often, providers do not disclose in advance what the cost to the customer will be. They may give some clues such as "we charge $100 per hour, depending on the complexity of the job", but often and especially with medical services contracts you have a relatively open-ended obligation to pay for their services. When the contract isn't explicit about the required payment, the courts will limit the charges to whatever is reasonable. That varies, depending on whether the service is lawn-mowing vs. contract-drafting.
A contract is not invalidated if a party is imprecise about the compensation that they are offering. Instead, the court would reason that Mr. Fogarty, being a sensible person, would not agree to pay the kid down the block $1,000 for mowing his lawn. There might be a consumer-protection law in the particular state that requires service contracts to disclose minute details about contracts, but in this case this is not a consumer contract (unless in that state the legislature has deemed Task Rabbit type workers to be "consumers"). So it could easily be legal.
However, your question seems to be about what is legal for the website, not the parties to the contract. This website simply connects potential parties to a separate contract. Actually, in ever job that I have held, the compensation was not specified in the advertisement, it was a matter settled between me qua service-provider and the employer. Sometimes advertisements hint at pay range, usually that info comes later, and it is completely legal to offer to treat with no hint what the compensation is.