Yes if you receive money for promoting prostitution in most states it is a felony with potentially many years in prison.
In some states, this crime is expanded to include the sale or distribution of textual or visual material openly advertised to appeal to the recipient's sexual interest.
The issue was discussed in the Craigslist situation where they took down the adult ads in fear of criminal charges. Their defense was they had no active participation in what was said in posts and would therefore argue they had no part of the solicitation. But a web person who designs an ad for a "prostitute" would seem easily to be a panderer (also known as pimp). Further if the money was deposed in a bank etc., its probably felony money laundering.
Now, if your client only offers something legal, like companionship with no prostitution terms like GFE or other sexual terms and there is never any mention of money for any sex act - just an hourly rate - there is probably no pandering nor prostitution.
You might still get arrested but if you pay $10K-$25k in legal fees you probably could defend against it and maybe get the prosecutor to dismiss. But if goes to trial fees legal fees could be much higher - like $100k+. If you can not afford the legal fees you would be entitled to a public defender.
However probably 99% of ads and websites are easy busts for both solicitation of the gal and pandering for the creator. In Phoenix escorts are busted for solicitation solely from their ads but may not be as aggressive in other cities.
Rarely if ever, have small web site creators been charged. This may change with the huge funding to stamp out consenting adult sexwork under the "trafficking" sham which includes consenting adults. This and the "rescue industry" is getting $millions in funding nationwide.
escorts.com owner convicted because they took money which was determined as a prostitution racketeering illegal enterprise. Eventually pled down to felony money laundering. Sentenced to 18 months probation and about $6.5 million in fines and criminal forfeiture of cash found in accounts money from escorts.com was deposited into or transferred from often using the rare underage prostitute as the emotional plea.
It is unlikely depending on how aggressive your state and local LE is that they would go after a small website maker. However, if its for prostitution its a serious crime (uniquely in the U.S. vs no big deal in almost all the rest of the world).
If anyone goes to the prostitute from another state, the FBI has jurisdiction but most likely it would be a pandering or money laundering state felony depending on your state.
