As others have said, the simple answer is these days, don't but as someone who did set up from scratch 15 years ago, here's a guide to what is involved:
Financial Standing: For 2018/2019, it's around £7600 for the first disc and around £4000 for every other disc after that. These funds have to be readily available to be drawn upon and have to be in your back account for about 6 weeks before your O Licence application goes in. It used to be 3 months but it has been reduced recently.
Operating Centre: This needs to be arranged before you submit your application. Costs vary between parts of the UK and what sort of set up you want. As a minimum, you will need access to water and electricity, preferably on a secure site with CCTV. As a start up, you will be better to negotiate 'parking' rather than a yard lease and just pay on a weekly or monthly basis. That way, if all goes wrong, you can walk away without massive lease commitments to pay for years to come. Expect to pay around £30-40 a week for parking per bus, more for undercover.
Insurance: The killer and what is killing most operators off now. As an example, we stuck to rail replacement, private hires and shuttles etc. No express work, no home to school and no stage carriage. Fully comp comes in about £2800 a year per bus. On top of that, you'll need Employers and Public Liability cover for around £10 million which is around £300 a year per bus. If you venture into stage carriage (ie bus routes or anything that is registered) expect your fully comp cover to increase to anything up to £7000 a year per bus. The personal injury claims are responsible for this as the higher risks involved in this type of operation are now colossal.
Road Tax: Single Decker around £330 a year, decker about £550 a year. Lower if you have RPC vehicles (which is highly unlikely as a start up as these basically mean Euro 6)
Vehicles: However much you want to spend. Example: 2005 Volvo B7TL single doored and with a test will set you back around £12-14k for a decent one. Repaint around £2000. Retrim around £3000. Hanover LED destination displays are about £2000 new or about £700 on ebay. Brand new Euro 6 deckers are around £210,000. Volvo B7TL will return around 8-9 mpg with a 5 speed ZF, around 7-8 mpg with a 4 speed Voith and don't entertain a 3 speed Voith. Roughly it costs about £300 a tank for around 270 miles approx.
Maintenance: Always budget at least £500 a month per bus plus the cost of the actual inspections by your provider. They'll be around £80-160 depending on whether you are using a local garage or a main dealer. Allow £1500 for each MOT. Tyres, if owned are about £400 each for decent Michelin or £200 for Chinese imports that also do the job but check the ratings if you plan on using the motorways regularly as some tyre ratings aren't suitable for motorway work.
Wages: Drivers - depending on which part of the UK but in the south and south east (outside London), if you want to keep insurance premiums down and quality levels up, then you will need to be paying around the £12 an hour mark as a minimum, more for call out work. Remember to add at least 15% employers costs to that for NI, payroll costs, holidays, sick pay, pension contributions etc.
Overheads: On top of all that, you'll have the costs associated with any business. Broadband, phones, website, advertising, printing and stationary plus lots of bank charges for your overdrafts, account fees, credit card interest, HP or loan interest. Then your accounts and tax return costs for accountants. It really is endless.....
Hope that gives you an insight and might also help explain why more operators are bailing out than ever!
Having said that, if you are dead set on setting up, I would never say, don't do it. Just plan, plan and plan even more. Make sure you have a niche to fill and keep everything as tight as possible.