ummmm....less expensive?? LOL I wish I could be of some help, but so far I haven't found any money saving tips other than making the sun shine more on the colder days. If anyone knows how to make that happen, PLEASE LET ME KNOW!!!
One tip I will give you (which has nothing to do with saving money...) is if you're building your greenhouse just big enough to hold the plants you already have, make it bigger!!!! I've heard so many times and experienced it myself: "always build bigger than what you think you'll need". Sadly, I learned the hard way.