For really hot weather (ie, North Carolina in May-September with a chunky 6mo), I found that the coolest were those in which baby was not directly against my skin: so a pouch sling or ring sling. I used my pouch sling a lot when ds was that age and able to sit up kangaroo-style, but had trouble finding one that fitted me properly, so never really used it with dd at all.
Apart from the sweatiness factor, my go-to carriers were/are my Kozy mei tai (wore dd in that continuously from her birth--in NC--in March through the Mississippi summer, and was fine with it) and the Ergo once they're bigger, esp for back carries. I hated every wrap I tried--too hot, too much fabric, too unwieldy and not enough support for my chunky babes.
As for cost, I'd look on craigslist (got to move fast, though) or consignment stores. Around here, there are often Ergos (for eg) in the consignment stores at a more reasonable cost, though they go quickly. I got mine in '06 when they were $75 new, which I thought was outrageous at the time; now, they're a lot more than that (though prettier, too!). If you're crafty, you could make yourself a pouch or a mei tai quite easily, I'd imagine (not being at all crafty myself).
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