The University of Massachusetts, in the midst of a budget crisis and steep tuition hikes, is paying $3.15 million a year for new downtown digs for President Martin T. Meehan and a posh alumni club — nearly double the cost of its old offices, documents obtained by the Herald show.The lease for 66,000 square feet of space at the prestigious One Beacon office tower is $1.5 million a year more than what the university was paying at its former Financial District offices and will eventually climb much higher, according to figures reviewed by the Herald.