This is unfair to whomever picks last, Say there are 20 items. In the first round, the first guy gets the best item, the second guy gets second best.
In the next round, the first guy gets the best item, the second guy gets second best. And it's the same until the items are gone.
Better that you decide in advance which of you will divide all the finds into two stacks. Then the other guy gets to pick his stack. It's like the story about mom making one kid cut the pie in half, and the other kid gets to pick his half. Keeps the divider honest!
And if the finds are a gold ring and 20 coins, instead of one guy getting the ring and ten coins and the other guy just getting 10 coins, one stack would be the ring, and the other stack the 20 coins. Not 50-50, but better than alternate choices.
(Or, you could agree in advance to sell any valuable finds and splitting what you get,
OR you each bid on the valuable item and the winner pays his bid to the loser.)
What if the ring was really his, or a family member, lost and forgotten, or even sort of the reason he want you to search? Would he value the ring as something more than melt value? Lots of coins may have a greater value, after you've gotten all the crud off, and see the mint mark, date, or production error. Fresh out of the ground, you might have trouble identifying just what you are holding, other than a coin, a penny, maybe a dime.
$8/hr is basically minimum wage, doesn't sound too bad, compared to the value of the junk I've been digging these past two months, little over $2 worth of coin. I'd jump on it, besides he might let you have something you really like, if he's happy with you work performance. I'm never expected to get rich, or find awesome things, just a little thrill, when finding something of value, even just a copper penny.