Translation:If that guitar was not that expensive, I could buy it.
I think that, were you to open a modern descriptive, as opposed to prescriptive grammar of the English language, you would find that the preterite form "was" is now common usage. For example In "A Student's Guide to English Grammar" by Huddleston and Pullum describes this use of "were" as "an untidy relic of an earlier system".
What if it's just a conditional? I.e. we're considering two possiblities: the guitar actually was expensive or actually wasn't, and concluding that it's the latter: the guitar really was not that expensive (as opposed to us only wishing for that). And in that case, yeah, I could buy it (where "could" just expresses less certainty that I'll follow through on my plan to buy it).
Fairly sure but not 100% to be honest. (1) "The guitar was not that expensive" is fine but "If the guitar was not that expensive.." seems dubious to me.
When in doubt, I tend to try a few different sentences with the same syntactic structures. (2) The sentence was that difficult I couldn't translate it - dubious (3)The sentence was that difficult that I couldn't translate it - even more dubious (4) The sentence was so difficult I couldn't translate it- fine (5) The sentence was so difficult that I couldn't translate it- fine (6) If the sentence was not that difficult I could translate it - dubious (7) If the sentence was not so difficult I could translate it - fine I checked in the "Longman Grammar of Written and Spoken English" ( a corpus-based grammar) and although it does say that "that" can be used as a degree adverb (as in (1)), I could find no reference to it being used as such in sentences like (2), (3) or (6).