I own a lock off, not at a marriott ski resort, though. Doesn't really matter, though. You will have the $90 or so yearly II fee and it will cost you $75 to lock off your week with Marriott. I'm not aware of any other fees, except for the II trade fee of $140. Someone may need to correct my number, as the trade fee may have gone up, but I am not more than $10 or $20 off, if anything.
As far as your plan to get a summer hawaii week in flex. Good luck. I guess that has been happening more lately, but you just never know. I have a year of owning Marriott and being in II behind me and what I have learned is that what matters most is having a platinum marriott week (from any resort and any size unit) that matters the most. Next, you need to be exchanging like size. Sure, you can deposit a platinum studio and ask for a platinum 2 bedroom in return and II will happily take your exchange fee, but your odds of landing an exchange are not that great. Thing is, all the other Marriott owners (even those who deposited lesser weeks, like gold weeks) who deposited larger size units instantly jump ahead of you in II. Flex works different but lately, the best flex exchanges have been showing up only a week or two before check-in. How crazy is that.
With all that said, you can still get great up trades. For example, I got a 2 bedroom 4th of July 2012 week at OceanPointe with just the one bedroom side of my 2012 4th of July Willowridge deposit. Got that request by calling II and asking for it. With the one bedroom portion of my summer 2011 Willowridge week, I got a 2 bedroom at SurfWatch for this coming Easter. I have used the studio portion of my Willowridge weeks for instant exchanges and done very well, pulling a May Planet Hollywood Las Vegas 2 bedroom and a one bedroom 4th of July OceanPointe week. I have always seen every II sighting with just the studio portion of my Willowridge week.
I share that with you just so you can see that if my Willowridge platinum week pulls that well, your Timber Ridge should pull just as well. Good luck.