moderate to low priced hotel near Borghese Gardens

Start at booking.com

What a wonderful spur of the moment trip! I'm not sure how close it is to the Borghese Gardens, but I love The Beehive. It is a hostel/hotel with many room options and the prices are great. I've stayed there twice and will stay there again in April.

I highly recommend booking directly with them.

Why near Borghese Gardens and which side? It's an awfully big park. You could stay in Parioli, which has some less-known hotels, more local, but it is about 20 minutes on foot to the park's east side.

The area you are looking for is not the cheapest area of Rome, on the contrary the Via Veneto and the Tridente are the most expensive.
Do a search on Booking.com using the filters with the amenities and price range desired, but I would consider other areas that are not as pricey. Trastevere, the Jewish Ghetto, Monti, Gianicolo are all very well centered in the historical center and yet not as expensive. The area near the station is also not as expensive, although it's not my preferred area. However the area between the station and the Viminale Palace is good.

Thanks everybody for your ideas. I'm meeting family and they already have hotel reservations in the Borghese Garden area and I want to be located nearby. Their hotel is booked-up.

Plug the address of your friends hotel into Google maps then do a search nearby for hotels.

As said above the park is big, better to know exactly which side they are located near

We stayed at Domus Australia. A lovely convent north of the terminal .
It is a 17 minute walk to the Borghese gallery