I rented from Hertz about six months out of the year in 2013 and just about constantly so far this year. I drive close to 1,000 miles each week to get to my job and renting was the only way to go. A lot depends on the type of vehicle you're renting, but I've been getting a small SUV in the winter months for around $700 per month. I've also had a Prius for around $570 each month. The Prius is considered a premium "green" vehicle in Hertz' fleet so a compact or intermediate would be cheaper. Although I haven't rented this class of vehicle, I have seen rates not too far from $500 per month if you rent far enough in advance and have a AAA discount.
A lease has a lot of downsides in my opinion. I looked into this too before renting and the following were the downsides for me (you're situation may be different): mileage restrictions, wear and tear (lease can charge you extra for "excessive wear and tear"), having to pay for separate insurance (AMEX and a few other credit cards have primary collision coverage that covers rentals), higher insurance rates (your personal insurance from another owned vehicle carries over to your rental; no extra cost), maintenance on leased vehicle (most car rental companies take care of oil changes and other maintenance for you). Also, if the rental company vehicle has a serious failure, it's nice peace of mind to call up the nearest location and trade vehicles.
I hope sharing my situation is helpful is determining the approach you want to take for your wife's new vehicle.