ROA is the annualized fraction of …
YourRewards / YourStake
Cost is a very generic term and most likely in this context refers to the cost paid by the delegator to gain YourRewards.
If you run your own stake pool, this would be the effective cost of running two Cardano nodes (i.e. hardware/hosting, domain, electricity, working hours, etc.)
If you delegate your stake, this would be what the stake pool charges before your reward is payed out. Cost comes in two flavours: fix cost (e.g. 340 ADA) and pool margin (e.g. 2%)
A comprehensive analysis of cost for the operator is here and for the delegator here.
If you like to maximize your ROA, you would want to minimize your cost. Ideally, you would want to delegate to a pool that only charges what is actually needed, which is IMHO well below 6 x 340 x 1.8 => $3672 p.m.