How can I take out a comma from a string and make it only as one number?

It's most likely something string.match does automatically when separating sets. You can simplify the string pattern you're using by using gmatch:

local str = ""

for i in ("Cost: $2,501"):gmatch("%d+") do
    str = str .. i
end

print(str)

You could replace the comma using gsub and then match for any digits. Like so

print(string.match(string.gsub("Cost: $2,501", ",", ""), "%d+"))