I did some calculations which put me off the idea of ever buying into a step, these are the results:
Step 1: buy-in inc. fee: $3, actual prize pool value: $12.05, effective fee: $1.80 (60%)
Step 2: buy-in inc. fee: $10, actual prize pool value: $48.21, effective fee: $5.18 (52%)
Step 3: buy-in inc. fee: $30, actual prize pool value: $174.74, effective fee: $12.53 (42%)
Step 4: buy-in inc. fee: $70, actual prize pool value: $476.03, effective fee: $22.40 (32%)
Step 5: buy-in inc. fee: $170, actual prize pool value: $1337.69, effective fee: $36.23 (21%)
Step 6: buy-in inc. fee: $500, prize pool: $4700, fee: $30 (6%)
If you want to confirm this, write out the steps as a system of linear equations and plug it into a solver (or do it by hand if you don't trust computers).