answers are to the best of my knowledge:
A. who pays expenses varies. if the athlete is still in college, the university might pay. if the athlete has some type of sponsorship deal, travel expenses might be a part of their sponsorship (oftentimes this is the case with shoe contracts and/or club sponsorships). otherwise, the athlete is most likely paying his/her own way. HOWEVER, if the athlete was in the top 50 in the world last year in his/her event, usatf is giving the athlete $1k to partially reimburse their expenses. athletes who make it to the finals might get some additional reimbursement.
B. if you are talking about money from usatf, yes it's the same event to event. if you are talking about sponsorships, the amount definitely varies from event to event. a premier athlete in a "glamour" event will usually get a better contract than a premier athlete in a less-well-known event. although this seems unfair, it's the same in other sports. do quarterbacks and centers get the same contract? or does the higher-profile position OFTEN pay more? i don't know the comparison male-to-female, but i do know that it can vary a large amount event to event. also depends on how marketable you are/make yourself.
i don't know anything about 3 so i won't even hazard a guess.