Almost half the American population does not see a dentist regularly, according to American Dental Society president Jospeh P. Cappuccio, and one of the reasons is that many cannot afford it.

Low-cost dentistry is not something easily found. Some people appear to be shopping for dentists solely on the basis of their fees, since the cost of even routine treatment varies widely in the Washington area.

For example, one downtown dentist charges $15 to $20 for cleaning teeth while another only a few blocks away charges $30. A dental clinic advertising in the newspaper lists $10.

One dentist may charge $250 to $275 for a porcelain crown, and another $375. For 1978, the Health Insurance Institute found that the cost of a one-surface silver filling averaged about $13 in the Washington area, but went as high as $20.

The discrepancy in prices is a result of many factors including the dentist's location (more expensive in urban areas), the quality of materials used, the dentist's reputation for providing quality care, and whether the dentist is new (with high start-up costs) or long-established.

Like almost everything else, dental costs have continued to climb, but, says Cappuccio, not as fast as many other items in the economy. New technology such as high-speed drills and the use of dental assistants has aided the dentist in seeing more patients and keeping fees down. Overhead costs for an office can run 50 percent or higher, says one Washington dentist.

Local public-health programs offer some inexpensive dental care in school and neighborhood clinics -- mostly for children, the elderly and occasional emergencies -- but facilities are limited.

Both Georgetown University (432-1234) and Howard University (636-6457) have dental clinics open to anyone where work is performed by advanced students under the supervision of instructors. The cost is substantially less (about 75 percent less, says the Howard dental clinic) than one might pay elsewhere.

Georgetown accepts appointments for new patients by phone only on the first working day of each month. Howard has stopped making appointments until Aug 20.

Georgetown's School of Dentistry has joined with SOME [So Others Might Eat] to open a clinic at the SOME office at 71 O St. NW (332-2220). Senior dental students perform routine and emergency work under the supervision of Dr. Gregory Lakas, a Georgetown instructor. The clinic saw about 230 patients in its first month. The cost is minimal, so the clinic depends a great deal on donations, which it welcomes.