Astrid,

Liquid nitrogen *is* pretty cheap - and knocks 200K off from ambient quite well.

The only other way to save costs might be to run a cryocooler on the article of interest - but they have modest powers, and are quite expensive.

Perhaps one could use a vortex tube and a suitable thermal link to the object that you need to cool? It might take off a few tens of K, and save a little cryogen.

https://en.wikipedia.org/wiki/Vortex_tube

Prof. James Garry is right Dear Astrid Altuve Nitrogen is the cheapest one. Also ask the IVIC low temperature group in Caracas. They might still have Helium and probably Nitrogen as well. What happened to the Nitrogen facility in Mérida?

To understand your problem correctly:

1) It all depends how low a temperature you want to achieve ?

2) Secondly what are you trying to cool (liquid, solid, powder, ceramic etc.,)

3) What are the dimensions of your sample which you are trying to cool.

4) Lastly, do you want to control the cooling/heting rate, just cool the sample to a certain temperature.

Once the above things are known, it will be convenient to suggest a quick cheap method.