With rentals its not as much the cost as the logistics. It'''s pretty tough to get a rental near me for a trailer and for a box truck they charge mileage and I'll be doing about 500 miles round trip whenever I need the cargo trailer.
I ended up buying a cargo trailer today. I got a good deal on one that should last a long time and given how well they seem to hold their value I don't think it'll be too bad to own. It'll double as a storage shed when not in use (which is something else I needed to buy). The cool part about using it as a storage shed is its not a building, so town setbacks don't apply and I can put it where I want (which happens to be by the garage next to the property line). Should work well... though I still like the idea of an enclosed option on my equipment trailer..