Congrats, and good luck.
The story will be what was damaged internally. If all it did was eject out through the lower case and chew up the trans, the parts bill won't be too much (assuming you buy an eBay engine and steal a lower case 1/2 and trans).
In the less likely event something got flung the other direction as well, there's a case wall, crank, and crank journals directly in line with the hole....
I'm gonna guess you popped the circlip, sheared or shattered 4th, and ejected the 4th gear chunks. 4th output is, by far, the weakest gear in the system.
Hopefully you had some of my luck with breaking things where secondary damage didn't occur.