In the pediatric dental office I worked for (and I will mention that between the two dentists we had over 11,000 appointments per year), we recommended that children come in for their first visit about 6 months after their first teeth erupted - usually around 18 months - unless the parents had noticed something dentally that they were concerned about, in which case we would see patients earlier.
At that age, the emphasis is mostly on making sure parents know how to take care of their children's teeth, and what to do if the child sustains a traumatic injury to the teeth (as well as giving them someone to call if that happens - most dentists will not see a patient for the first time as an emergency, you already have to be an established patient). The other purpose is to acclimate the child to having their teeth examined, so they can learn that it isn't scary (and can be fun!)
At our office, we almost never did x-rays on patients under 4 or 5 years old unless there was a reason to do so (such as the child said they were experiencing pain, or the doctor saw something suspicious during the exam). We also did not do actual cleanings on children under 3 (just brushing). So the expense for a child that young should not be much - 2 visits per year at about $60-$70 each (at least until they're old enough for a cleaning). When children are old enough for fluoride/x-rays, the cost does go up significantly, but at least in our office we would only do x-rays once per year, and only billed for fluoride when we had not done x-rays. So it was still usually under $200 per visit.
I definitely think it is worthwhile to establish routine dental care as part of your child's expectations early on (my son loves going to the dentist and sometimes spontaneously asks to go), and it's also really important to have someone with whom your child is familiar (and vice versa) in the event of a dental emergency. That might partly be based on my own experience of not having had that - I fell and broke some teeth when I was a child, and because I had patchy dental care, it wasn't properly treated and developed into an infected maxilla which I then had to deal with long into adulthood. In terms of cost/benefit analysis, I'd say it's definitely worth it. But take the time to find someone you can trust to be good.