There is an Ibis in Wenceslas Square and - according to where I think it is on the map - it's about 300 meters from the venue.

https://secure.ibishotel.com/gb/hotel-3195-ibis-praha-wenceslas-square/i...

I just booked the rooms on their website on a 3-for-2 special offer which worked out at €128.64 (about £118) for Friday-Monday.

There is also wireless internet at this hotel which is not marked as a "paying service" so I assume it's included.