Hello.
Cement is differently something that speculated a lot with here.As it is Paraguayan cement factory's are working at 100% out-put since Idk when, but are still unable to satisfy the local market 100%. Just one crusher or one mixer goes off-line and the fun starts all over. Just be on the lookout for contraband cement. One can really buy rubbish if one doesn't look out.
I was part of a house building project last year, it was a 100`m house with 3,5m wide veranda right around. Our aim was to do it with G120.000.000 ($28k) but we didn't make it, got closer to G150.000.000 in the end.
I would guess pine is here too the lightest and cheapest, they just call it pino.
As for prices, a suggestion from me, try Hierropar, never dealt with them myself but they provide cement, bricks, and earth's AND metals.
As for a builder to help you build, good luck with that one.
Another suggestion from me, for your roof, if its possible, why not think of doing it with metal instead of wood.Same price, (if not a bit cheaper by now) and it won't bend with time.