"Billig" usually doesn't have that connotation unless you say something like "Den ser billig ut" to imply that it looks flimsy or poorly made, or pair it with negative nouns ("billig skrap"). It's quite neutral.
However, we do have "rimelig" as an alternative, which is closer to "affordable" or "fair", and can be used as a more positive version of the neutral "billig".
When used attributively, meaning that it's placed before the noun, it's declined like this:
"Billig" is for indefinite singulars of all genders. (other adjectives have gender-specific forms too)
"Billige" is for plurals and definite singulars of all genders.
"en billig skjorte" (a cheap shirt)
"den billige skjorten" (the cheap shirt)
"flere billige skjorter" (several cheap shirts)
For predicative adjectives, adjectives positioned after the noun, the rules differ slightly. "Billig" is then used not only with indefinite singular nouns, but with the definite singular ones as well:
"Skjorten er billig." (The shirt is cheap.)
"Skjortene er billige." (The shirts are cheap)
in the course of lessons I found somewhere in the clothes section mannen vasker skjorta si - but if the singular is skjorte and skjorten, as you mentioned before, what is skjorta for? i stopped learning for months and now that I'm back its a little confusing trying to catch up with new additions you've made.