Smoke from neighbors coming in with new baby on the way..

When we checked out the place there was no hint of cigarette smell. Once we moved in we noticed it every now and then. Now it's getting colder the guy has been smoking inside constantly. It makes me feel nauseated, and we have a little one due Dec 3rd. It is frustrating too because the owner has some special agreement with the guy because his dad used to own the property , and he has lived there for several years now. We were thinking of an air purifier. Is there one personally recommended for smoke that's not a grand? There must be one that would be effective but not too spendy. We've been here 2 months and don't want to break the lease.