I want to have a house built.I need to know how much of a proportion
of what the house will ultimately be worth I should consider paying
for the land alone?

It depends on what size and quality (structural & finish) that you
want. Determine a size and price per sf that you want to pay for the
house, and then look for land to build it on. Land is available in all
price ranges. If you find you can't locate suitable land, you'll have
to scale back.

If you are considering building on a lot in an already developed area,
I think you should avoid building a house that is significantly
"better" than the neighboring ones.  For example, the price of your
dream home only leaves you with enough money or credit to buy a lot in
a "lesser" neighborhood.
A house and lot are an investment, even though most persons building
tend to think they are going to live (there) forever.  The neighbor to
a house I know "overbuilt", and the house is now on the market for
much less than it cost.

Your question is justified.  Remember the three rules about real estate -
location, location, location!
A house can be improved, but the location seldom does (beyond general
inflation in the area).

I hope Cynthia, who is in the real estate business, agrees.

Isn't there a rule of thumb for the relationship:  value of lot to house?

Yes, myoarin, you're right. Location is important, but even builders
have an idea what they want to build before they find the land. I
suggest princessga spend some time looking at new home development web
sites in Michigan, looking at the price per sq ft of homes offered,
then comparing them to lot size and location. Look in city and 1 hour
away. See what the same price gets you in each area to try to
understand location and cost of the house per sq ft.

Forget the Michigan reference, that was from another question (duh),
but she should look in her city, wherever it is!