I’ve pulled up the 2007 MassCap info from the condition book (which is put together by the racing secretary and lists the conditions [type of race –claiming, allowance, stakes– restrictions –non winners of two, fillies & mares, maidens, etc.-- distance, track surface –dirt, turf, or all weather– and so on so forth] under which races are to be run, a little like a prize list for a horse show. I’d have posted 2008’s, but that block of the condition book isn’t online just yet.) Hopefully this will give you a better idea of how it works:
SEVENTY-FIFTH DAY – Saturday, September 22, 2007
Entries Close on Wednesday September 19, 2007
…
11 ELEVENTH RACE HANDICAP STAKE
The 66th Running of the Massachusetts Handicap
$500,000 Guaranteed
A HANDICAP FOR 3 YEAR OLDS AND UPWARD By subscription of $100 each by Monday September 10, $2500 to pass the entry box and $2500 to start. Weights: Saturday, September 15. Starters to be named through the entry box by the usual time of closing. Trophy to winning owner, trainer, and jockey. If this is not divided it will be limited to 14 starters. ** The money to be divided as follows: ** 60% of all monies to the winner, 20% to second, 10% to third, 5% to fourth, 3% to fifth, and 2% to sixth. To the winner, any horse who has won multiple Grade I or Group I Stakes at a mile or over in 2007, and additional $200,000. Any other graded stakes at a mile or over in 2007, an additional $100,000.
Nominations close Monday, September 10, 2007
ONE MILE AND ONE EIGHTH
So, for last year’s MassCap, conditions were $100 to nominate, $2500 to enter, and $2500 to start…for a total cost of $5100.
Obviously the MassCap is not restricted to fillies & mares. Generally speaking, trainers want to enter horses into a race in which the horse stands a good chance of winning or at least hitting the board, and usually it takes an exceptional filly to beat the boys at the stakes level.
Keeping that in mind, note that there was a bonus offered in addition to the purse – clearly an attempt to draw bigger name stakes runners, which in turn help attract more people into the Grandstand to watch & wager. Should a filly or mare with enough graded stakes earnings (and therefore, most likely possessing a bit of “celebrity status” along with it) want in to the MassCap, methinks she wouldn’t be turned away. 
Nominations for the MassCap in recent years have been announced in industry publications. For example: Blood Horse article listing all horses nominated to the 2004 MassCap. There were 35 horses nominated that year; last year 34 horses were nominated.
'Hope that helps.