In terms of fees, there are bargains around, but the two you have cited are exceptional. The law averages means if you spend £600k on a player, he will be far less able than a £7m player.
Lambert & Sinclair also signed for clubs who were actively pushing for promotion and had budgets to match...
Also, a model based on selling your best players may balance the books, but will not gain palace promotion.
You only go up by keeping your best players adding to your squad.
Palace finished where they did with zaha, apparently a player worth more than £7m. If he goes, at best you'll replace...