Diapers are the least expensive at
Amazon, using a free Amazon Mom membership and Subscribe & Save. Like everyone else said, see what kind of diapers fit your LO best, and that they don't have a reaction. My son got a terrible rash from Huggies and Luvs within minutes, and could only wear Pampers. On the other hand, my daughter doesn't seem to have such sensitive bum skin.

She can wear anything, but Pampers Cruisers definitely fit her the best.
I used to get great deals on wipes at Amazon, but not so much lately. Our favorite wipes are Huggies Naturals unscented, but we use various unscented brands if the price is right. Sam's Club always has a decent price of Huggies Natural wipes. The only baby wipes that I truly dislike are Pampers Soft Care, because they're very slimy. I made the mistake of buying an entire case of them without trying out a smaller package. We ended up using the whole case for just cleaning up after meals, because they weren't good for nastier jobs. Another inexpensive option are the Parent's Choice unscented baby wipes from
Walmart, if you buy them by the case of 700.
Here's something that you'll notice about the major brands of diapers. Pampers has 2 different types of diapers - Baby Dry and Cruisers. (They also have a Swaddler diaper for young infants.) Baby Dry diapers are a little less expensive, and you get more to a box. Cruisers are a bit more expensive, and you get a little less to a box. (Cruisers are a more "structured" diaper than Baby Dry, with more elastic around the legs. Plus the absorbent padding goes up much higher in the back.) The stated weight range on the package is exactly the same, but I've found the Cruisers are made quite a bit larger than the Baby Dry. The Cruisers stretch a lot more than the Baby Dry version. My 34 lb 2 year old toddler wears a size 4 in Cruisers, but I have to buy a size 5 in Baby Dry. Of course, as the size of the diaper goes up, you get less per box. So, even though the Cruisers diapers are technically more expensive than the Baby Dry version, I am able to buy a size smaller - which means I get more per box, and they end up being a bit cheaper.
The above also applies to Huggies Snug & Dry and Huggies Little Movers, although the size difference between the two doesn't seem quite as much as what the Pampers are.