There are sooo many ways on how to depopulate PCBs, one could write almost a book on that ;-)
Using a heat gun vs the return of RAMs is not costly at all. It goes very quick, if done right, since it takes only a few seconds per board. I wonder why you estimate the costs so high?
If you have time and hard work is not an obstacle, you can use a chisel, or even an electric chisel. Some members have done that and it works well.
A third very easy but not environmentally clean method is to dip the boards in dilute Sulfuric Acid. This will also dissolve lead and much of the tin. All you need to do is wait and the components will fall off.
One disadvantage of the heat gun are the problematic fumes. That is why I do not use this method very often anymore. PCBs contain hundreds of chemical substances from the manufacturing process, resins and more. So ensure to have a very good ventilation if you do it that way.
Edit: Why on earth are you using a temperature of 700 °C ??
That is really dangerous, because the PCB will "gas out" and toxic fumes will be created.
The melting point of tin alloys are lower than 350 °C and some alloys (Bi) are even around 140 °C. Maybe that is why it takes you so long and so much energy.
Instructions: Preheat the PCBs to let's say a bit below 100 °C, then quickly give them a shot with the heat gun with a max. temperature of around 350 °C. All components should fall off within 5 seconds.
700 °C is far too much! A strong blower will just blow the heat (and the fumes) elsewhere.
Use a slow or medium stage of the blower, not the strongest one.
You can add vibration to speed up the depopulation.
I have developed such a machine, when I worked as an engineer in an electronic manufacturing company in the early 2000s. At that time there were no other solutions known to me.
Today there are very simple made "machines", I would not call them a machine btw, from China. They consist of a stainless steel drum and a large heat blower. The drum turns and hot air is blown into it. as the tin melts, it drops trough small holes in the drum to the bottom. PCBs and components remain inside the drum until the cycle has finished. You can find them on AliBaba, but they are so simple, you can build one by yourself (I guess)