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I think it's very easy to find a cheap place to in or near Hallum, the VVV (tourist office) locally in Leuwarden can book hotel, pension, camping farm, or even a private room locally (usually).

There are several lists for "budget lodging Holland" and of course the local "skater network" usually is able to host visitors from other countries.

You could ring up Henk Schra at Stouwdam , maybe he could set you up with a contact?

There are several Dutch skeeler forums you might ask on, as well.
In the small towns of Friesland it sometimes is difficult to find a "regular place" to stay. However there are many "mini camping" farms that usually have a room to rent (especially in skate-crazy Fryslan).
Nearest city to Hallum is Leuwarden, I stayed there many times and easy to find some budget lodging there. Hallum is about 20km away and your race probably starts at noon or later so you could stay in Leuwarden and bus trip it out (there may even be a rail line)
Here's a link to bed and breakfast inns nearby Hallum (note-every small hotel serves brekkie with the room)
http://www.bedandbreakfast.nl/land/Nederland?tab=2
Here's a link to the closest b and b to Hallum:
they have 2 rooms for rent and one apartment (probably the room has the toilet down the hall, but usually an in-room sink
http://www.leafesawntjin.nl/english/index.htm
BTW take your soft wheels and prepare for bad weather, or at the very least strong wind - Holland is pretty flat (completely flat in Friesland) but the wind, and the klinking bricks you skate on when the race goes in and out of villages, makes up for the lack of hills)