I just went there during early June and it cost me around RM1500 (incl. AirAsia return tickets @ RM500 - coz i didn't manage to get a cheap flight ticket, trip to water rafting, manukan island and Mt. Kinabalu and we stay at backpacker's lodge) for 1 week.
For Mt. Kinabalu trip 2 days 1 nite it cost me around RM350. At Mt.Kinabalu, trip to stay at Laban Rata coz there are heaters in the room and u have restaurant over there.....for other resthouse, u need to cook by yourself.
For porter, they charge RM8 per kg....and only let porter to carry those stuff that u r not using while u r climbing, coz porter don't climb together with u..they will walk first. If those stuff that u need to use like camera and accessories but too heavy to carry, u can always negotiate with your guide..coz they are willing to carry for u so long as u pay them some "coffee money"
On the weather, nowadays very hard to predict. Everyone said May and June is the best time to climb Mt.Kinabalu, but the first i reach Sabah...it's heavy rain
but lucky that 2 days i climbed mt. kinabalu it didn't rain but the cloud is quite thick!!! so didn't really manage to see sunrise!
For your info...the guide can stop u from climbing if there is heavy rain.
u can visit
www.sabahtravelguide.com for more info. and do book early if u r going there during holiday season..coz it's going to be quite crowded!!