In a perfect world, your employer would jump at the chance to send you, give you full per diem and a room in the conference hotel, rental car, and an allowance for books and materials on sale at the conference.
But as Huey Lewis said, "Ain't no living in a perfect world."
I was fortunate to go to Black Hat and Defcon in Las Vegas for 11 years while I was at my previous (private sector) employer. They paid for all but the first time. For that one, I took leave, paid my own way, and then came back and demonstrated to them the value and knowledge I picked up (mainly by starting just about every sentence with "Well, in a talk at Black Hat..." I got laid off when the company was downsizing, ended up in a public sector agency, which sounds very similar to your situation (great people, interesting work, surprising lack of sticks inserted up people's butts). Same situation - I had to go on my own first, the next year they willingly paid for me to go.
Your employer is at least offering to pay for the training piece, which says that they see some value in this. And I know how hard it is to do things like this on a public sector salary (which is still about 40-50% of an equivalent private sector one). My advice: look for the bargains. Stay at a cheap casino (you can get into places like Excalibur for $40-50/night, sometimes lower) instead of the conference hotel. Walk and use the monorail to get around ($10/day). Eat fast food, or fill up on conference munchies - don't eat in the conference hotel or celebrity chef restaurants, but find the coffee shops and cheap buffets. And most of all, talk to your employer. Tell them you're willing to go on your own dime this time, but when you get back, you'll want to make the case for someone from your group going every year, fully paid.