Slave prices are necessarily estimated absent an actual slave market. Even then, outside extraordinary cases only the slave caste gets bought and sold, so slave prices of peasants, freemen, &c would still need to be estimated, although we would at least know the critical determinants of slave prices.
However, if you were genuinely doing it properly, each person would be a publicly traded corporation. Simply tot up the market cap of $ALRNS and there you go; that's how much I'm worth. A minor share would carry an obligate dividend, based on your earnings, exactly as with fictive corporations. Perhaps a wage job could be formalized as a significant share. Slavery is merely when someone owns all your shares.
Thus we can easily formalize taxation. When you're born, the State arrogates a certain amount of shares to itself, something on the order of 30%. It return it makes promises to you it may or may not keep. If you make too much money it starts arrogating more of your shares to itself.