Downloading LOTS  

Yes nothing worse than getting charged for going over your limit and I understand it can be quite expensive.
Look for free wi fi hot spots as well as internet cafes, thats what we do from time to time. Free wi-fi hot spots are great,
We can park the car by a church in the city and have a choice of about 10 unlocked wi -fi aignals to use.

Maybe you should try to get another internet provider.
I know several people that have 2 or more internet providers
or just provider options at the same time.
And speaking of options you could change your subscription just for this month.
Or add more services of the same isp provider,
like some of cable or adsl providers,
also give that mobile usb stick for free,
wich comes with its own traffic and connection options.
Of course, if this is just this a one time need,
maybe asking a friend with unlimited bandwith is the way for you.
You can also look for services that will download the files for you,
these usually help those who are internationally capped.
they bring the files to a local host first, and then download it from there.

As said, and since you'll be downloading through FTP, finding some free Hot-Spots is probably the way to go since there isn't a tight enough limitation on most of them that would prevent FTP access.

Throttling down your speed seems pointless too, since it'd be just more clever to monitor the download and stop when you believe you have used enough bandwidth for the month while reserving yourself some for the remaining of it for normal surfing and whatnot.

In the end: Get some other ISP.
In this day it's almost hard to believe that there are still ISPs limiting the amount of bandwidth one gets available per month, although I suppose there's of these in every country, but thankfully my own did a severe mistake and advertised itself as having unlimited bandwidth forever... they had this little clause in the contract, half hidden away, that stated that there was indeed a limit of only 250GB (Upload and download) per month... but that made their advertising misleading and were sued for that. As such, no limits, yay.

Anyway, other than using Hot-Spots there probably isn't much that you can do. Guess your friends can lend a hand, but, do be sure to not try and hijack some other person's wireless connection; I'm sure that for leaving it vulnerable they just deserve it, but you don't really want to land a bill on someone else's expense, right?