Rental cars come in classes, but these classes are not quite the same with each rental provider, so check them carefully if you want to compare like-for-like.  Generally, the smaller the car the cheaper it is to rent.  

A compact class should also have the best fuel efficiency, particularly if you opt for a diesel.  However, if you intend going off the beaten track, or will be driving in snowy or icy conditions a 4-wheel drive may be needed.

Once you’ve ordered what you need you’ll probably be recommended an upgrade. But be aware that a higher speck of car in one class may be a better car for your purpose than the larger, lower speck option in the next class up for the same cost.

Just to confuse the issue further, as we’ve mentioned before there is a wide seasonal and country variation.  An estate or MPV could be cheaper at some times of the year in your destination country than a compact class vehicle.

Yet another part of the mix is whether to go for manual or automatic.  Manuals are always cheaper and can be less than half the cost of an automatic.  Manuals also offer better fuel efficiency.  On the other hand, if you aren’t used to a stick shift, and will be learning how to use one in an unfamiliar car and country, you may be safer paying the extra.

And as for campervans, are you sure you need one?  They are comparatively expensive to run, can be a pain to park and manoeuvre, and are slower than a car.  And if city sight-seeing is on your itinerary, then you may have to park further out and use public transport to get to the centre. It could make sense to hire a car and spend the money you save on B&Bs.
 
As a rule of thumb for budget car hire, try a manual, diesel compact class.  But do your homework, bearing in mind what we’ve mentioned above.