You will be in for around $3500 at the cheapest to "try" Open with a used pistol, used mags, holster set up, and reloading machine. I would go 9MM major which will cost you about 13 cents a round. If you decide Open is not for you, you will get $2500 or so back out.
If your tryout leaves you wanting to make shooting USPSA Open a big part of your life, you should get a top end pistol ($5000) made for you. You can change to 38Super(comp) or stick with 9MM major. With the latter, your shooting will be cheaper but your smith choices will be more limited. You will also need a Dillon 650 or 1050 if you didn't go that way out of the box.
I did the above and could not be happier. The high end pistol is worth every penny and makes moving into the higher reaches of achievement easier. Over the pistol's life, I figure it costs $1000-1500 a year. Unfortunately, no smith I know takes payments that way.