What am I doing wrong with this equilibrium constant answer

I am trying to work out equilibrium constant and can't see what I am doing wrong at all.

Keq = Products / Reactants

The reaction is between:

(CH2)3 (g) <--> CH2=CHCH3 (g)

The concentrations are:

(CH2)3 (g) = 1.1 x 10^-6 mol dm^-3

CH2=CHCH3 (g) = 0.10 mol dm^-3

so therefore, I've put the following:

Keq = 0.10 mol dm^-3 / 1.1 x 10^-6 mol dm^-3

and I'm getting:

90909.0909091

That number seems completely wrong to me - is it???

Your calculation is fine. It just happens to be a fairly large equilibrium constant, and it tells you that this reaction forms almost all product. This isn't surprising since cyclopropane (CH2)3 is not very stable.