If you think a new nut is costly, you probably will want an inexpensive substitute for the nut file as well. Look up Lincoln Electric tip cleaners at your local hardware store....should run you about $4, and do a stellar job when used carefully.
I concur with the
Stew Mac tip of Super Glue and baking soda (or sacrificial bone donor: old nut, or the saddle of an acoustic guitar)....First, tape off the fingerboard and headstock side of the nut to capture runoff, and maybe use tape to crate a mold form on the front and rear faces of the nut. Recommend you put the glue into the slot first, then immediately sprinkle the powder in gradually...prevents a dry clump of powder forming at the bottom.
To start the nut slot on a budget, I suggest a Nicholson 6" taper file (around $4 at your local Lowe's/Menard's/HomeDepot, also useful for fret filing, saddle shaving, etc.... will also help restore the rectangular shape to the nut faces after you've glopped the glue and powder in place.) Use the pointed edge of the triangular file to start creating the "V", then the appropriate size tip cleaner to round out the bottom (even cheaper trick is to use the snipped end of your old A string as a nut file....) Follow the conventional advice to be sure your nut slot slopes DOWN from the fretboard side to the headstock side....keeps your intonation tight.
Let us know how this turns out for you!