COLUMBUS, Ohio (AP) _ If nothing else, it's a lesson in Economics 101: University of Texas junior Chuck Falgout invested $21 in lecture notes and turned a possible F into a B in biology.

A ''subscription'' of 48 issues from a note-taking service helped the enterprising Falgout flesh out his understanding of a difficult subject. And with the help of the store-bought notes, he could even afford to miss a lecture or two.

In an age when it seems everything can be had for a price, lecture note sales are booming in college towns around the country, fueled by rising demand from underachievers and overachievers alike.

''We like to consider it a supplement to your own notes, but there are a lot of students who skip class,'' said Criss Dolan, manager of Paradigm Notes in Austin, Texas, which sells the written word on everything from anthropology to economics.

Paradigm enjoyed so much success in its 14 years that it opened stores in Arizona and Illinois and inspired a former customer to open a shop in Columbus.

The companies pay graduate students and college seniors with good grades to attend classes and take notes for the publications.

''There are some professors who say, 'Paradigm notetakers, don't put this in your notes,' and we honor that,'' said Dolan.

That allows a professor to slip in a key point or reveal a test question to students who do attend class without actually punishing those who skip, she said.

Kathy Gatton, 26, said Paradigm helped her get through school and four years later, she opened Grade A Notes across from Ohio State University, home to 60,000 students.

''It's one of the largest universities and it didn't already have a note- taking service,'' said Ms. Gatton, who expects to triple her business and earn her first profit this university quarter.

Grade A opened in September of 1987 and earned modest revenues of $60,000 in its first school year. This month, the company expanded its note offerings and nearly doubled the size of its office.

''Business has been great ... (but) we still get students who come in and say 'Gosh, I didn't know you were here. This is great,''' said Ms. Gatton, who plans to franchise in 10 more big campus towns in the next five years.

Most of these services sell notes by the class for about $2 or by subscription, for $18-$24 per quarter or semester. The note takers have 24-48 hours to turn in their type-written notes. Most are paid $10 to cover a 50- minute class session.

''It's great money. I wish I had the GPA (grade point average), I'd be a note taker,'' said Erik Hemp, 23, an office employee of Notes & Quotes, which opened in the fall of 1986 near Arizona State in Tempe.

Some educators feel the service encourages students to cut class. On most campuses, the note-taking companies are required to obtain professor approval before sending someone in, but the universities generally do not have official policies on the matter.

''It's not banned by the dean. I would encourage (instructors) to ban it and would never allow it in my own classes, but it never came up for formal discussion,'' said James Casey, dean of the College of Communications at the University of Illinois in Champaign with 38,000 students.

It did, however, come up for discussion in the history department at University of Texas.

''It (Paradigm) was thrown out about four years ago,'' said Brian Levack, acting chairman of the department. ''We thought it discouraged people from coming to class and therefore we expressed unanimous departmental disapproval.''

The companies' main advertising vehicles are word-of-mouth and leaflets circulated outside of classes.

''That's part of the territory and we never leaflet on Friday. Kids blow off the classes then,'' said Hemp.

Note takers earn most of their business in large introductory lecture classes.

''Problem-solving classes aren't suitable for note taking,'' said Gary Magee, owner of Notes & Quotes in Champaign. ''We're pretty limited in scope.''

University of Texas biology professor Stephen Adolph is so fond of the idea, he turns over his own lecture notes to Paradigm.

''I haven't found a good textbook for my class so it gives them something to fall back on,'' he said. ''There are a lot of business majors here who aren't interested in biology, but want to do well. You can get an A in this class without attending.''

He said many professors disapprove of the service because it's disheartening to look out into a lecture hall that seats 300 and see only 50 students.

''Sometimes it's a blow to the ego,'' Adolph said.

End Adv Weekend Editions Nov. 5-6