The compulsory registration now costs nearer to 200 Sudanese pounds (60 or 35 Euros) as of a month or two ago. I did it in Kassala. Those who did it in Khartoum say the same.
I tried all the options for Khartoum accommodation, though didn't stay at the Blue Nile:
I highly recommend to try the Youth Hostel, 35 per person (6 euros if you choose a good 'bank' to change money at) for shared rooms, great location, secured shaded parking under a huge mango tree with a sweet old guardian, clean and well run, air con and fans. Even a working fridge in myroom and a (dirty) shared kitchen to use. Loads of good restaurants etc around. Easy to walk to the centre etc.
Rooms have between 2-6 beds in them, I am sharing a 3 bed room with my 'wife', so we effectively end up with our own room for a decent price.
GPS N15 35.478 E32 32.386 (Only Landies and Toyotas will fit through the gate/trees - trucks will not fit!)
Sudanese Youth Hostels Association
We tried camping at the 'National Camping Residence' which is nice, clean toilets and showers, but hardly any shade and its a long walk to anywhere being right on the edge of town, over 10km to the Nile etc.
Khartoum is bloody hot (1st week of March here = 43 degrees in the shade for a week, it has calmed down to 37 now) so definitely worth spending a little more - the Youth Hostel works out at being cheaper than camping at the Blue Nile (constant bad reviews) and is quiet, the Blue Nile is noisy etc.
If no suitable rooms are available couples may have to sleep in separate rooms as it is strictly separated by sex per room etc
Worth a try, I'm enjoying the electricity for my laptop, cold fridge, pizza from the Deli next door and the gentle hum of aircon after a hot day of mechanics.
For internet I bought a Zain sim card for 5 Sudanese pounds for my smart phone, added some credit and then subscribed to their 3g internet service at 1 pound per day. Fast and easy. When I want to use it on my laptop/usb modem you can usually get them to work by dialling *99# using 'internet' as the username and password. Fast in Khartoum, fine everywhere else....