Work backwards from the price you'll actually get after the marketplace commission. If a customer pays 1000 rubles and the commission is 15%, you'll receive 850 rubles. From that amount, subtract the product cost from your supplier and your own expenses (if any - advertising, internet, payment system fees). What's left is your profit before taxes.
The key thing is don't mix up shipping. In dropshipping through WB, it's usually included in the product price or calculated separately by the supplier, but comes out of their pocket. Check your supplier agreement to make sure they won't be charging your account for logistics. If shipping is on you - deduct that too.
Before you add products, open a spreadsheet and calculate a few items: purchase price → marketplace price → minus 15-20% commission → minus expenses → remainder. If the remainder is pathetic, the product isn't worth it. A normal margin in dropshipping is at least 20-30% of the marketplace price, otherwise it's not worth doing.