You can drop a factory $65 9mm barrel on your .40 and replace the mags and it will work just fine. You can do a search on here to find others that have done this.
The cost for the barrel and one mag will be $100 after shipping. If you save 3 cents per round you should recoup your costs after 3,333 rounds. If you buy two mags it will take you around 4200 rounds. If you buy a Storm Lake conversion and a couple mags it may take you well over 6,000 rounds to recoup the cost.
If you are paying more than 24 - 25 cents a round for .40 you should shop around.
Now if you just want to buy a 9mm and have another gun, that's the BEST idea by far! All costs go out the window because you are making an investment! A lifelong investment!