Before I try to answer this, a quick FYI that Sound Devices can answer this question in less than 24 hours. Fill out a Support Inquiry on their webite (
http://www.sounddevices.com/support/inquiry/) and they'll get back to you within a business day. Now for my 2nd hand attempt at botching this answer....
I think the cost would depend on the age of your unit. If your 744T has a serial number greater than 461509071000 (or 471309092000 for 722 users) , it will already have a SATA interface connector for the HDD, however older units were initially built with PATA (aka IDE) connectors, a much older technology for which only a few niche-brand SDDs have been brought to market. The XL-SATA interface retails for ~$120 and I imagine SD will charge you a pretty penny for whatever SDD they install vs. typical market prices, but that's to be expected because they're going the extra step to validated and stock a specific SDD make/model/size that will play nice with the 744T.
If I had to guess, I bet you'd be looking at $300 in parts, maybe a bit more, for SD to perform the upgrade and I don't know if/how they would charge you for labor if the upgrade is done at the same time as a general checkup.
If there is nothing notably wrong with the unit and the checkup would simply be for piece-of-mind, I might recommend that you perform the upgrade yourself. Sound Devices provides clear instructions on how to perform the upgrade and consider it to be a user-serviceable part. The upgrade is dead-simple and takes all of 20 minutes. Doing it yourself means you can save a few dollars on the actual drive and possibly get away with installing a larger capacity than SD would recommend.
The risk to doing it yourself is selecting an SSD that is somehow incompatible with the 744T and from my own experience, this is a real concern. I have TONS of old SSDlaying around now; lot's of 40GB and 80GB intel drives that are no more than 3 or so years old and they all work perfectly with the 744T, but I recently tried to install a much newer 120GB Intel SSD and it simply wouldn't take. The 744T kept telling me there was an I/O error with that drive my best guess is that the controller in the SSD simply isn't compatible. Sound Devices is incredibly cautious with SSD upgrades for the 744T because they know that the much older hardware of the 722/744T simply won't work with newer, large capacity drives.