Bitcoin price is based on the market price. All cryptocurrency exchanges have prices that vary slightly for all cryptocurrencies. You can check sites such as tradingview and coinmarketcap for the current cryptocurrency prices
There are no guarantees in the cryptocurrency world. The price we sell or buy bitcoin or other cryptos is determined at the time of the trade, whether it goes up or down is at the mercy of the market. You can monitor market prices using various services online such as trading view to make your own technical analysis.