Ryanair flight from Venice-Treviso to Düsseldorf-Weeze is just 30.49 Euro. Flying Friday to Germany and Monday back.
From the airport to Essen, you take the bus shuttle there and back. Each trip is 15 Euro. You can also take the train, but you'll have to change two times and it costs around the same.
To stay you need a hotel for three nights (for the flights above) and the cheapest hotel room we found so far is in Gasthaus Nordstern (north star inn) for just 26.50 Euro per night. You can also get 4 star hotels below 50 Euro/night.
For conference participation ASH charges you 199 Euro. If you are student, there is a discount. Also if you do a short or a long presentation (and there is room in the schedule for you), the price may be lower.
So on the end, I bet you can make it for less than 400 Euro.