[Jimrod]

If Dexter really is a gambler (who knows what he says is truth any more) he chose an amount most likely to get him big returns. If another housemate chose under 50k then he could win more by winning the show (a gambler always expects to win). If the other housemates wanted the cash they would most likely go for as high an amount as they could get away with presuming others had chosen the highest amounts possible to stay in the show - this is what the twins did. Dexter bid high enough for large returns but low enough to likely be under the others.

Nothing to do with knowing twists or lucky numbers but simple gambling odds. He wanted the cash (as did the twins).