I think a few of the existing resources on this website will already answer most of your questions.  For example:

 - http://photo.stackexchange.com/questions/21368/will-cokin-z-and-hitech-100mmx150mm-filters-fit-on-a-lee-holder
 - http://photo.stackexchange.com/questions/112/what-is-the-highest-quality-graduated-neutral-density-filter
 - [Marumi ND2-400 Variable ND Filter Review][1]
 - http://photo.stackexchange.com/questions/306/what-nd-filter-would-you-recommend
 - http://photo.stackexchange.com/questions/2327/how-do-cokin-and-lee-filter-systems-compare-to-each-other
 - http://photo.stackexchange.com/questions/4510/whats-the-difference-between-singh-ray-vari-nd-and-cheap-fader-nd-filters
 - [\[filters\] tag][2]

I believe the above questions answered all of these:

 - Does all filters that match the Cokin holder will fit in the lee holder? 
 - Assuming that the answer to the first question is yes I would like to get a recommendation for a good solid ND set of filters (1,2 and 3 stops).  
 - Does lee's ND filters worth the high price?
 - Is there any other cheaper set of ND filters that can produce similar results (in terms of quality)?

So that leaves us with:

 - What are the differences between lee's polyester, resin and glass filters? 

This is answered by Googleing the exact same question, which leads us to this article at the [Lee website][3]:

> LEE Resin filters are individually hand made, the 2mm Optical Resin is cast in our factory, hand dyed and cut to give a very high quality product both in colour accuracy and optical quality. They are hard wearing and ideal for use on location. Polyester Filters are made on a master roll, the colour being coated onto the surface of the clear polyester base material. Because the filters are very thin (0.1mm) the optical quality is high. Polyester is a good, low cost alternative to Resin, but because of the manufacturing process polyester grads are not available. The Glass Filters that LEE offer tend to be specialist filters that are not available in other materials, such as the enhancer or polariser. They are hard wearing and scratch resistant but tend to be expensive. Do not drop!

  [1]: http://photo.blogoverflow.com/2012/03/marumi-nd2-400-variable-nd-filter-review/
  [2]: http://photo.stackexchange.com/questions/tagged/filters
  [3]: https://leefilters.uservoice.com/knowledgebase/articles/23017-what-is-the-difference-between-resin-polyester-