It can be expensive, but I always look for sales! Toys R Us best sale is buy one get one free. Sometimes, they have buy a figure, get a pack free OR buy one, get one 40% off OR buy 2, get a 3rd free. But I think for power discs (which are completely OPTIONAL and you don't have to buy them if you're looking to reduce costs), BOGO free is best sale. For figures, BOGO free or B2G1 free is fine. Also, check Best Buy, Target, and Amazon.com because they'll have sales too, and if you have ads, you can probably get one to match the other (except Amazon).
If you just buy the starter set, I think it sometimes drops to $60 (instead of $80?) and I think groupon is throwing in the cars playset (which is $35?) and a pack or two (retails $5-6 each) for free. If you just buy the starter set, you get 3 lands you can play with.
Depending on how many figures, playsets, and discs you personally want will be the cost/expense. If you don't like Lone Ranger, there's a playset you don't have to get.
Etc. My friend got the Toy Story and Cars playsets and after he was done with them, he's loaning them to me to play them (so I don't have to buy them).
If you have friends like that, you can split expenses between each other.