Your ratio would not work very well since you have fixed offsets.
Unless of course your usage is so high that the offsets become negligible.
What I'd do is to calculate the difference in cost of the suppliers COST1 - COST2.
If it is positive supplier 2 is cheaper.
The resulting equation is:
COST1 - COST2 = 20 - 2.904 H + 4.256 L
The result comes out like this Wolfram plot.
The break even points are the points on the line in this plot.
If you really want a ratio, we would need to neglect the 20 term and set the difference in cost to zero.
0 = - 2.904 H + 4.256 L
L/H = 2.904 / 4.256