I think you'll find they weren't trying to rip you off, the challenge with group travel is that group airfares are almost always more expensive than the standard discounted fares you see advertised, but along with that higher price comes benefits such as flexibility with names and payment you just dont get on a discounted fare.
You really need to organise a group to appreciate the complexities they bring and why its better to use a group fare even though it may be one or two hundred more than the cheapest available fare.
The other issue (and the reason I suggest they werent attempting to rip you off) is that even if you do choose to use the cheaper nett fares when organising a group booking, its almost impossible to get enough seats on the cheaper fares to get everyone booked on at that rate...sure you can book yourself and question why you could get it yourself for $1800, but I guarantee theres no way they could have got everyone on at that rate, hence why they didnt use it and opted for a group fare.
Unfortunately without understanding some of the complexities and restrictions you face day to day as a TA its easy to sit back and question whether a TA might be trying to "rip you off" etc, but i'd suggest alot of the time there's other reasons behind it rather than simply a lack of ethics.
TG