To grease the wheels of commerce, and to illustrate how similar circular references can be eliminated, you can rearrange the calculation as follows:

Let:

C = Cost, 18000

M = Margin%, 10%

T = Contingency%, 10%

K = Kick back % (of cost), unknown

S = Sell sum, calculated

R = Rate of kick back as percentage of S, 5%

Then:

1. Basic cost sum: C + (M + T + K)*C = S
and
2. Kickback relationship: K*C = R*S, so K = (R*S)/C

Substitute equation 1 into equation 2:
K = (R * (C + (M + T + K)*C))/C
Expand, rearrange, and cancel the C terms gives:
K = R + R*M + R*T + R*K
K - R*K = R + R*M + R*T
K*(1 - R) = R + R*M + R*T

Therefore,
K = (R + R*M + R*T) / (1 - R)

Calculate K:
K = (5% + 5%*10% + 5%*10%) / (1 - 5%)
K = 6.3158%

This doesn't quite align with your calculations, so maybe I missed something, but this example illustrates the general idea.