Just go and get a hi point in what caliber you want, then if you need to defend your self you will not have alot of money in and the DA can keep it for long as he needs it.
I keep a Taurus 605 .357 in my truck, cost less than $300. A Mossberg Shockwave 12ga. is great if you have room and under $400. There are a lot of good choices under $400.
In the $400 range, you have plenty of solid options for a truck gun. Every firearm I've ever kept as my truck gun has been around $400. My current truck gun rotation is an M&P M2.0 .40 cal that I got for around $380 brand new and a Sig P250 45 cal that I picked up for about $410. Before that, I kept a 1st gen M&P 40 in the truck. Paid $325 for that one. And before that, my truck gun was a Walther P99 that I paid around $420 for. I could go on, but I think you get the point: You have endless options. Good luck! Let us know what you get!