I've never booked with Tui myself but I've had the occasional look and I’ve definitely seen Royal Pacific as an option before. Either way it can make sense to book one night at one of the three top hotels even if you have another hotel sorted for the duration of the trip just for the express.
August is peak season and would be pretty last minute, but I've just had a look at the night of the 13th August on hotels.com for two adults and two teens. Royal Pacific is £403, Hard Rock is £476 and Portofino is £488.
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If we go with Royal Pacific as the cheapest of those, that works out to £100.75 for two days of unlimited express per person / £50.38 per day and you get a hotel room chucked in.
If you were buying Unlimited Express direct from Universal it would cost $180 + tax per person on the 13th and $170 + tax per person on the 14th. That works out to $372.75 / £293.47 for two days of unlimtes express per person / £146.74 per day.
The only problem is once you get used to having Unlimited Express you'll always want it. My wife and I love Sapphire Falls but tend to end up staying at Royal Pacific instead just to get the benefit. Then again we don't have kids so only have to cover flights / park tickets / meals for two, which makes the whole trip much more affordable. We also collect Virgin points which we use for flight redemptions etc.
For a little while we had the top tier Universal annual pass which gave us single use express after 4pm, and when we had that we ended up staying at Sapphire but this year we went back to lower tier passes and booking Royal Pacific. Being able to do what you want, when you want, with very little time spent in the lines is very hard to beat.