Land costs what it costs, regardless of what's sitting on it. When the value of the land as vacant exceeds the value of the land and the structure on it, you will generally get a tear-down situation. An improved site, i.e. utilities at the property line, level and rough graded, zoning in place, etc. will be more valuable than raw land, but what you put on the site doesn't change the value of the land.
It's all about highest and best use. If you have a 900 square foot, 2-bedroom house built in the 1950s on a one acre lot in San Francisco, the land is not at it's highest and best use. If it were vacant somebody would probably try to build an apartment complex or office building or something that would make better use of that land. That land might be worth $1,000,000 either way. If it's got the old house on it, it will probably sell for right at a million because the cost to demolish a small old house is a rounding error at that point, maybe $5k. If it had an old concrete warehouse sitting on it, the value might actually be lower because the buyer would discount for the higher cost of demolishing that, maybe $50-100k. If it has a 6-story apartment building on it, the whole thing might sell for $10 million, of which $1 million would be allocated to the land, and $9 million to the improvements.
Developer's expected profits can be calculated several ways. I knew one developer who built Walgreen's drug stores. He expected to make $250k in profit on each one. Generally the land for a prime corner lot would cost about a million, construction of the building would be around $3 million, and then it would sell for somewhere over $4 million.
If you're building a subdivision from raw land you have a much greater risk so you will expect a higher profit, at least 20-30% annualized. If you're building a cookie cutter fast food restaurant or retail that will quickly sell on the secondary market (drug store, auto parts store, etc.) you can't expect as high of a profit, maybe 10%.
It's the same as any other market, risk vs. reward. The greater the risk, the higher the reward someone needs to make that investment.