There is a formula for valuing land, which works in most cases:
Land costs (A) + Build Costs (B) + 20/30% Margin (A+B x 20/30%) = End Value (C).
example land cost £200,000 + example build cost £300,000 + 20% margin £100,000 = end value £600,000
Always do your sums. You can look up the average build costs tables in various self-build magazines, you can also refer to Potton's Budgeting Guide available to download here or pick up at the Potton Show Centre.