I currently have two different kinds of air purifiers (3 of one kind and 1 of another) and they use a minimum of electricity. In fact, I think my ceiling fans use more than the air purifiers do.
The best air purifier, in my experience, is the Oransi Max HEPA Air Purifier. it costs @ $480 and the 3 filters, that have to be changed out annually, cost about $100/set. After buying and disposing of 12-15 different brands over the years (because they didnt work well), I've now had these Oransi air purifiers in my bedroom, computer room and living room for the past 6-7 years. I'm still amazed at how well they work.
One air purifier that's inexpensive but works fairly well is Oregon Scientific Air Purifier. It costs only $120 and the filters are only $20. I bought one of these recently (because I like Oregon Scientific products) and have it in a second bedroom, where it's working quite well. Check its reviews below: