Advertisement prices are based on Bitcoin exchange rates. The Bitcoin exchange rate is volatile and may change rapidly.
Remitano updates its exchange rates and advertisement prices every few seconds to minutes. Prices shown in the listings and on the front page are cached, and update a little slower. Sometimes when the Bitcoin price is fluctuating rapidly, advertisements with the same price equation may display different price. Sometimes market data is not available for some currencies, which leads to delays updating the advertisement prices. However, when you open up the advertisement page itself the price will be more up to date.
The price of the purchase is determined at the moment the Bitcoin buyer sends in the trade request.