You are trying to use the string as a function:
"Your new price is: $"(float(price) * 0.1)
Because there is nothing between the string literal and the (..) parenthesis, Python interprets that as an instruction to treat the string as a callable and invoke it with one argument:
>>> "Hello World!"(42)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: 'str' object is not callable
Seems you forgot to concatenate (and call str()):
easygui.msgbox("Your new price is: $" + str(float(price) * 0.1))
The next line needs fixing as well:
easygui.msgbox("Your new price is: $" + str(float(price) * 0.2))
Alternatively, use string formatting with str.format():
easygui.msgbox("Your new price is: ${:.2f}".format(float(price) * 0.1))
easygui.msgbox("Your new price is: ${:.2f}".format(float(price) * 0.2))
where {:02.2f} will be replaced by your price calculation, formatting the floating point value as a value with 2 decimals.