A quick search shows that most hotels in the area are fully booked on that date, with £199 being the cheapest on offer, several hotels asking for £340 and some seeking £430. So it seems obvious that you'll need to stay elsewhere.
I should have asked you how many people are going to the event. (You put 'I' in your first sentence but then referred to 'tickets', so it's not clear whether you'll be alone or with one or more companions). So I don't know whether to look for a single room, twin, double or whatever. However here are a few suggestions anyway:
Tony's House Hotel in Paddington (from where you can walk to Marylebone Station reasonably easily) has a single room with shared bathroom available for £45.00 or an en suite double for £109.00.
[Use the Hostelworld link above]
The Metro Hotel (also in Paddington) has no single rooms but a twin room with shared bathroom for £87.56, a double room with shared bathroom for £86.70 and a double en suite room for £99.00.
[Hostelworld again]
The Marylebone Central Travelodge (convenient for Marylebone Station) has rooms (for either one or two people) at £136.00.
https://www.travelodge.co.uk/
The Elmwood Hotel is near St Pancras, from where you can walk to Euston Station easily enough. A single en suite room is £75.00, with a double en suite at £110.00.
[Hostelworld]
I hope that there's enough there to give you some options to consider.