What will you use it for? Sergers are great for home dec (such as curtains) and clothing (especially children's knit clothing) and doll clothes. If you don't intend to use it for clothing of some kind, I'm not sure it's worth having one.
If I were going to buy a new serger, I would definitely buy jet air threading but that does cost a lot of $$$.
I found one of these sergers sold on eBay for $85:
http://www.ebay.com/itm/BABY-LOCK-BA...item3cd1fc3818