-아/어야 can vary a bit in meaning and nuance (from simple "if" to a dismissive "even though", as in 후회해야 소용없어 = 후회해도 소용없어), but what is crucial in this example is how tightly you bind 좀 and 비싸야.
'좀 비싸야' here is similar to 여간 비싸야. 좀 or 여간 in these phrases means "up to a point", "within limits" or "reasonably", to make the whole sentence "If it were reasonably expensive I would buy it".
좀's most common meaning is straightforward "a little" or "somewhat" though, and it is often used as a prompt for action (e.g. 저 좀 도와주세요). If you take 좀 in this sense and treat it like a phrase level adverb rather than bound to 비싸야, the sentence becomes "If it were more expensive, then I would buy", which doesn't make sense as you noted.
I think the example is a bad one in that it can mean two opposite things depending on how you interpret 좀. The phrase is also traditional and idiomatic (it is a construct well-worn through usage over a long time), and this makes it a hard one for learners to understand. In written form, using phrases like 여간 in place of 좀 would leave no possibility of ambiguity (while others like 웬만큼 and 어느 정도 (both meaning "to a certain extent") would have a similar issue as 좀). 여간 is used strictly in negative or conditional phrases just like this example to create the effect of "within reason" or "tolerably". For instance, 여간 비싼 게 아니야 = It is not reasonably/tolerably expensive = It is awfully expensive.
[EDIT] I think this type of tight or loose couplings occur pretty commonly in most languages. Here's an English example.
- I'm not going to say no to you because you declined to help me the other day.
- I'm not going to say no to you, because you declined to help me the other day.
#1 is saying "I'm not going to say no even though you did that", whereas #2 doesn't make much sense because the declination would only serve to make people less helpful. #1 has the because clause coupled with the verb "help" (just like 좀 in 좀 비싸야 is bound tightly to 비싸야) but #2 has it working with the whole of the main clause. The meaning changes depending on to which part you connect the sub-clause.