Prices always have been around what you've searched during summer, its peak tourism season. However, some companies like Enterprise boost theirs a tad more. Renting a Kia Forte from Enterprise, for 4 days, is 500$ taxes excluded.
For suggestions, you could always stay on the western part of NB, Transcanadian Highway from Edmundston, to Fredericton then Moncton. But, the northernmost part going around Campbellton, the Baie Des Chaleurs, Bathurst and Miramichi takes longer but offers better scenery (if you plan on ending your trip in NS).