Less expensive than what? If you mean less expensive than booking direct on the website , generally no, in fact some airlines charge you a fee for that.
There is often a fee of 25 to 50 to work with a CS rep. I always book online and it's very easy. No wait time for the phine either. For you don't want to do it yourself online than I would rather pay a TA to help. I always check out flights in Expedia or kayak first and then go direct to the airline website to book it. I would never book via one of the online TAs because then you totally lose control of the reservation that way in case something goes wrong
I know JetBlue has a fee to book on the phone. I'm also extremely paranoid about everything and am scared they will misunderstand me or I won't get a confirmation email and will think of 500 other things that could go wrong so it'd be more stressful that way for me.
Delta eliminated their over the phone ticketing charge in April 2016.
So if OP is considering booking with Delta, it won't cost extra to call Delta and book.