Its really simple. Look at what a game sells for whole. Then ask whats unique about it? vector monitor, unique controls, unique cabinet, laserdisc, ect?
Then figure about 1/3 of that cabs price is its board. If its loaded with unique parts, then figure about 1/3 of that for the parts(controls). If it has a unique monitor then about 1/4-1/3 for that, if its a unique cab then 1/3 for that. Often, then the sum of its parts is worth more than the whole for unique games. If the game can be easily emulated, then the board will be worth less. if the board cant be easily emulated, it might be worth more.
so look at what 720 sells for. What unique about it? Unique controls (1/3), unique cabinet parts and lots of unique artwork (1/3), and the pcb(1/3). The medium rez monitor is not that hard to find(1/6). So if a 720 is a $1500-$2k+ game, then it stands to reason the board should be worth $500-$700 working. not working(AKA unknown) figure about 1/2.
what makes things complicated is when your talking vector games. a game like space duel is a $500-$750 game But it has a monitor in it thats worth $300-$500 because it can be a donor to other games that are worth more.
paperboy is an interesting example. it has very unique controls. So much so, the controls are worth 1/3-1/2 of the games value! It has a unique monitor, and a somewhat unique pcb.