Ok I found a solution for my scenario.
I combined those two plugins to filter by custom taxonomy and custom field:
Query Multiple Taxnomies http://wordpress.org/extend/plugins/query-multiple-taxonomies/
Custom Query Fields http://wordpress.org/extend/plugins/custom-query-fields/
I changed the QMT Plugin template by adding the possibility to choose several custom fields and do a combined query.
The URL now looks as follows:
/?qmt%5BTAXONOMY1%5D%5B%5D=1532&price_min=400&price_max=600/
Greetings!
I recently enhanced my filter, making it a little bit cleaner, faster and less complicated as you dont’t have to edit any plugin code. If you are interested, just get in touch.