C# is a strongly typed language so you can't add decimal types to string types. You need to convert your input from a string to a decimal. Luckily in .NET that's pretty easy.
public void GETEXTRABEDCOST(int APPLY, Decimal EXTCOST)
{
decimal COST = APPLY * EXTCOST;
decimal currentValue;
if (decimal.TryParse(txtextrabedcost.Text, out currentValue))
{
txtextrabedcost.Text = string.Format("{0:0.00}", currentValue + COST);
}
else
{
}
}
See also
Standard Numeric Format Strings | Microsoft Docs[
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