Hi Cap,
The one day passes must be bought at Disney...and I believe at the gates before you enter the park. (meaning I'm not sure if you can purchase them at the resort as well as the parks) They cost $58 and change...that includes all taxes.
Two things...if I were going to buy them at the gates I would do this before your in-laws arrive so you won't have to waste time standing in line on your in-laws park day.
Second, I might consider buying a multi-day hopper on Hotwire, paying for it, and letting the in-laws use it for one day. That way they can pay you the $58 and you will have bargain ticket prices for your family left on the passes for future use.
It can save you a bundle if they were going to pay for a one day pass anyway.
[ex. buy the 7 day PHP (4 plusses included) for $302..which includes all taxes, shipping etc. Then apply the $58 (that they were going to pay Disney anyway) towards it. Voila! You end up getting 6 park days and 4 waterpark days which never expire...good any day of any year until you actually use them up...for $244) We usually figure that's 6 park admissions for $34 each and 4 waterpark admissions for $10 each. ]
I have done many variations on this for friends and family we take to Disney. Last October I brought 6 good friends (women's club group, most in their 70's) down for a week at Disney. None wanted to buy any extra tickets, but didn't know how many days they would end up doing Disney. To solve the problem I found the best available prices for one-day, 2 day, 3 day, 4 day, 5 day, 6 day and 7 day PHs (they didn't want to do PI or any waterparks). Then I bought 7day PHP's and told them that whatever they used up on the tickets is what they would be charged. They all all started out giving me the money for 4 day PHs, knowing that if they used less or used more (I'd refund or collect later) it would be according to the best price I could get for PHs for the number of days they used up.
LOL...they ended up using 4 days even though most of them thought they would only use 2 or 3 so I didn't have to refund or collect more...and they loved Disney!
Anyway, after all was said and done, I had 18 park hopper days and 24 waterpark days for about $29 and $10 each respectively. My friends were happy because they got the lowest price going for the days they used and had the luxury of deciding day by day what they wanted to do without having underbought or overbought tickets! I was happy since my husband and 3 teens only get to Disney at Christmastime anymore...and mostly would rather do the waterparks anyway after all these years.
Not for me, I say, clutching my treasured annual pass to my Disney addicted heart! lol
Anyway, it was just a thought since a one-day pass applied to a multi-day pass really saves a lot. Also...if you buy it ahead on Hotwire Savings Club (just do the 30 day trial for $1 and then cancel if you have no further use for the membership)...although a lot of people keep the membership (about $8 a month) and buy the maximum in gift cards at several different stores they shop anyway and save an additional 20% at those stores. (You pay $20 for a gift card and it gives you $25 to spend with it which you can use for everyday items). These can be used at Target, Kmart, JCPenney's etc...the site has the current list)
Oh...and you can also buy the Q&C and VKL vouchers there too and save even more.
LOL...this is LONG...and probably more info than you wanted or even maybe needed...but once I get going it's hard to stop!