Hard to say. I don't remember what size my daughter was in by time she trained, but I'm sure it was either a 4 or a 5 and she was about 2 1/2 yrs old and skinny.
My son is 25lbs now and turned 2 in May and wears a 4 in White Cloud and Huggies but 5 in Pampers and Luvs!!
There are size 6 diapers but I don't know for sure who makes them.
I understand what you say about diaper prices!! I noticed that our Walmart used to carry the cases of Pamper in size 5 with 80 diapers in the case and now it went down to 76 diapers and the price is still 21.74!!! All of Pampaers lowered their quanties in their packs!! Oh, and some people don't know this but Luvs is made by Pampers!! And for some reason, I've noticed that case quanties are larger at Walmart than compared to Kmart and Target in Luvs and Huggies. If you called Huggies and either complain or compliment their products, they'll send you some execlent coupons!! Oh, and don't buy diapers at the grocrey store-they are way more expensive that way and always buy them by the case which is cheaper!!!
Play around with store brands too. Some of them are really good diapers!! I like White Cloud but my son can't wear Parent's Choice because he leaks in them. Buy them in small packs first to try them out before making the dive into a case size. I've tried Target brand as well and didn't care for them, Kirkland (Cosco brand) didn't go over well either. The only one I haven't tried is Kmart's Little Ones.
Good luck and happy diapering!!
S.