Transaction fees are paid when transacting, not when trading, or at least not necessarily.
Assuming you're trading on an exchange site, you will have an account there. You can buy and sell BTC for other currencies, but all that is happening is that the exchange credits your account with what you buy, and removes it from the seller's account. This process does not involve any interaction with the Bitcoin network; it's all just updating numbers in the exchange's database of accounts.
The exchange may charge trading fees for offering this service, but the details of that would depend on what site/service you use, and the amounts involved. Some exchanges offer multiple fee tiers based on your trading activity.
There are transaction fees involved once you decide to withdraw your BTC from the exchange to a wallet you control yourself. The exchange may eat those for you, or they may charge you for them. Some exchanges may themselves charge a withdrawal fee on top of the BTC network transaction fee. The transaction fee is independent of the amount transferred; it depends on the complexity of the transaction, and that depends on how the exchange chooses to dispatch their withdrawals as Bitcoin transactions.
Once BTC are in your own wallet, and you want to use them, you will be the one to pay transaction fees. Typical transactions are around 150-250 vbytes in size. At 20 sats/vbyte, a 200 vbyte transaction would cost 4000 sats or 0.0004 BTC. Again, this value is largely independent of the amount transacted.