The cost of 100k Robux varies depending on the package and method of purchase. On the official Roblox website, the price for 100k Robux is $1,250. However, if you purchase Robux through a third-party website or in-game, the price may be different. The exchange rate for Robux to USD is approximately 80 Robux per $1, based on the 400 Robux package costing $4.99. To calculate the cost of 100k Robux, you would divide the number of Robux by the cost in dollars, which would be 100,000 Robux divided by $1,250, resulting in a cost of $0.00125 per Robux. This means that 100k Robux would cost $1,250. It's important to note that the cost of Robux may fluctuate over time and may vary depending on the region and method of purchase.