Well 100 Megabytes equates to 800 Megabits. Simple crude calculation says for one user it would take 26.666 seconds (ignoring any overhead for protocol and any bandwidth used by the server for any other purpose). In reality it will take a tiny bit longer, but the precise figure would be dependant on a few issues such as MTU at the other end, quality of line causing retransmissions, and whether the server may be using the connection for a purpose other than serving your file so I wouldn't bother getting that precise with the calculation.
Assuming the file is being served from a server which either has a decent RAID setup, is capable of caching the file or if you have a proxy such as squid caching the file (basically assuming your hd thrashing isn't an issue), you could simply multiply that 26.66 figure by number of users so:
1 User : 26.66 secs,
5 Users : 133.33 secs,
10 Users : 266.66 secs,
25 Users : 666.66 secs,
50 Users : 1333.33 secs,
100 Users : 2666.66 secs,
250 Users : 6666.66 secs,
500 Users : 13333.33 secs,
1000 Users : 26666.66 secs.
If you wish to adjust these numbers to figure out how much quicker it would be with more bandwidth, just use the relevant ratio to scale - e.g. these calcs are based on your 30Mb connection. e.g. If you could get it upgraded to 50MB, divide my numbers by 3 then multiply by 5.
You haven't mentioned the expected number of users, and ultimately you need to decide what is and isn't acceptable as we cannot decide that for you as you will have to trade off the cost of the connection vs the speed increase.