The usual shortcut for a round trip is to take the cost of getting in and double it. It is a fair first guess and it is usually low. The two legs are separate transactions, and several things can make them differ.
The venue can price each side separately
Nothing requires a venue to charge the same on a buy as on a sell. A fee schedule or a disclosed markup can state a different figure for each side. Where that is the case, the only correct round-trip figure is the sum of the two actual numbers. Doubling either one gives the wrong answer. Reading the disclosure for both sides is the fix, and it takes a minute.
The book is not symmetric
An order book has a bid side and an ask side, and they are built by different people. At any moment there may be more size resting above the price than below it, or the reverse. A buy walks up the asks. A sell walks down the bids. If the bids are thinner, selling a given size moves the price further than buying it did. Reading order book depth shows how to see the two sides.
The conditions are not the same
Entries are chosen. You can wait for a calm moment, a tight spread and a busy hour. Exits are often not chosen. A stop triggers when it triggers. A thesis breaks when it breaks. Those moments tend to be fast and one-sided, which is when spreads are widest and depth is lowest. Spreads widen exactly when you need them tight explains why.
So even on a venue that charges both sides identically, the realized cost of the exit runs higher across many trades. The calm exits and the forced exits average out to something worse than the entry.
The size is not the same
A position that has gained is larger at the exit than at the entry, in money terms. A percentage cost on a larger amount is a larger amount. This is a small effect on a small gain and a real one on a large gain, and it works in reverse for a loss.
Pools have their own version
On-chain, a token in a liquidity pool is priced by a curve that treats buys and sells alike. The exit still tends to cost more, because holders leave together and the pool is shallower when they do. Why exits cost more than entries on thin pools covers that mechanism.
The working rule
Estimate the two legs separately. Use the venue’s stated figure for each side. Then assume the exit happens in worse conditions than the entry, because over enough trades it will. The NoVo Crypto Market Map ranks the round trip on each coin from Robinhood’s own disclosed markup, which gives the stated cost of both legs together. The conditions on the day of the exit are the part no table can show in advance.