It's possible that he's just a late talker, but it's also possible that he needs more help. If you can help him now, it helps to reduce the risk that he will get farther behind or have trouble in school. Knowing the specifc problem is also valuable - if he had apraxia, for example, then specific help would be needed and he wouldn't just catch up. I don't want to worry you, because kids can do very well and catch up, but it some kids really need the help from therapy. You may find out that your child doesn't (maybe he will do really well if you work with him at home), but it would be a shame to miss getting the earliest possible help if he needs more than that.
You can generally get free therapy (or at least lower cost therapy) through Early Intervention. There are different names in different states. The Department of Human Services should be able to help, or you can call your school district (they provide services and evaluation for children over age 3). It's really worth looking into it because they may be able to help a great deal and even send a therapist to your house if that's easiest (my son had a wonderful speech therapist and developmental therapist all for free through EI and is now above age level in language skills - and it was all free).
If you did get an evaluation by the county (Early Intervention) and they said his delays weren't severe enough to qualify for services, then I'd suggest talking with the speech therapist who did the evaluation to find out exactly what would help your child the most. They may be able to give suggestions. If they aren't helpful, then maybe you could find another therapist who understands that therapy is too expensive and who will let you make occasional appointments to check your son's progress and get "homework" targeted to his needs without having to have weekly therapy.
As soon as he turns 3, he will be eligible for Early Childhood Services instead of EI. They have different eligibility requirements. You can ask for additional services. If your private speech therapy evaluation showed more severe delays than the EI evaluation did, then you can also take that back to EI and ask for help. See if they will accept a private evaluation to qualify your son for services.
Also call local universities. They may have reduced cost therapy through programs in which students are trained to become speech therapists. You may be able to pay a speech therapy student to work with your son occasionally as a less expensive option even if you don't sign up for regular therapy through the university. You might even be able to get free therapy if you sign up for a research study.
You can also get some good books to use at home. James MacDonald's Play to Talk is very helpful and the Hanen Program has a book called It Takes Two to Talk that is supposed to be great. There are a lot of things you can do at home to encourage language, but there are different things to emphasize depending on what is causing the problem. Even a single visit to a speech therapist might help you decide on the best approach to help at home. The therapy is play based, so your daughter can enjoy participating or helping out.
Yes, some kids do just catch up. I have family members who were late talkers and who were fine. However, I also know people who have children who needed a lot of help and who wish the problem had been caught earlier.