To give you an insight as to why you can't do a discotheque professionally for £120 without subsidising it from your daytime wage, here's the maths:

These figures are all worked out based on 52 discos per year covering 2500 miles in total at an average of a 30miles per round trip.
Both Vehicles are cars of approximately the same value. (you will not get much equipment in a car without a trailer )
Diesel at a cost of 137.8p per litre and Petrol at 132.3p per litre have been used in these calculations

An annual Standing Charge has been applied to both cars, this includes (Tax, Insurance, Cost of Capital, Depreciation and Breakdown cover), and based on 1/7 of a year (52days)
Running costs have been applied to both examples, these include (Fuel, Tyres, Servicing, Parts, Labour, Parking & tolls)
The following is a breakdown of what it costs to do one disco.

  Diesel Car Petrol Car Notes:
1. If disco uses a Transit sized van, cost will increase significantly.
2. Based on a 30 mile round trip.
3. Labour cost for travel time, rig/de-rig and playing.
4. Based on National Minimum Wage £6.16/hr.
Standing Charge per annum £524.556 £644.657
Standing Charge represented in pence/mile £0.21 £0.2579
Running Costs as pence/mile £0.2075 £0.2524
Total Vehicle Costs in pence per mile £0.42 *note 1 £0.51 *note 1
Vehicle cost to do one disco* note 2 £12.60 £15.30
8 hours labour/man *note 3 £49.52 per man *note 4
99.04 (DJ + roadie)
Music, PLI, Admin, Consumables, Other business running costs. £3000pa or£57.69/disco
Running total
£119.18 (DJ only)
169.33 (+Roadie)
£122.51 (DJ only)
172.03 (+Roadie)
  We have yet to factor in equipment hire cost and profit margin.

Source of vehicle and fuel data: Automobile Association (June 2012).