For independent travellers using the UK as a base, budget carriers (aka cheapo airlines) are usually the cheapest way to travel to most countries in Europe. It's possible to fly for as low as 1 GBP/ 1 Euro per leg of journey (though not everywhere and certainly not all the time), and it's even more common to see deals that offer flights for 10-20 GBP/Euro plus taxes and airport charges.
However, those enticing prices have a habit of creeping up after all the extra fees, charges and additional costs are taken into account: be a savvy traveller and avoid the unnecessary ones, while budgeting for the unavoidable.
1) Make sure that the price quoted includes airport taxes and charges. These vary by country and airport, but can easily add around 20 GBP/Euro to each leg of the journey. Some Ryanair deals waive the charges
2) Most cheap carriers now charge luggage fee, accrued per item of checked-in hold luggage. At the time of writing, Ryanair charges 10 GBP per first bag and 20 GBP per each additional bag up to the total of 15kg hold luggage allowance. Other cheap airlines charge between 5 and 10 GBP per bag.
3) Charges for excess baggage are very high: Ryanair charge 15 GBP per kg of excess luggage, easyjet over 20 GBP per kg. Please note that even passengers travelling on the same reservation are not allowed to pool their luggage allowance (i.e. one bag of 25 kg cannot be "shared" between two travellers).
4) Sports equipment (e.g. skis) are charged extra again, Ryanair charges 30 GBP per item (e.g. a pair of skis).
5) Cabin baggage allowances are often more generous than on regular flights: Ryanair has a 1 item of up to 10kg allowance, but requires that the total of cabin luggage fits within this allowance (and the required dimensions), including all purchases made on the airside.
The rule regarding luggage is is to check every time you travel, as allowances and charges vary by airline and change over time.
6) During the booking process, you will be offered various extra services (e.g. priority boarding and travel insurance). Remember to un-check all those boxes if you want to avoid extra charges.
7) Many cheap airlines charge a booking or card payment fee (often per passenger per booking), which depends on the type of card being used to pay. Almost invariably, booking by telephone is more expensive than on-line, and often payment by credit card is more expensive than payment by debit card. For example, at the time of writing Ryanair charge 12 GBP for credit card payments and 5 GBP for debit card payments. Easyjet charge 2.95 GBP for debit card payments and 6.95 for credit card payments. Payment by Visa Electron debit card is free with both of the above. Wizzair charge 4 GBP for credit and some debit cards and 2 GBP for other debit cards.
8) Ryanair charge an extra on-line check-in fee for each flight (currently 5 GBP), which is waived for their special deals (1, 5 and 10 GBP flights): this makes such deals even better value. It's crucial that you check in on-line, as a Ryanair airport check-in costs 40 GBP.
9) Financially speaking, most cheap flights are practically if not technically non-refundable It's possible to change flights, but this change incurs a fee (around 30 GBP if done on-line, more if by telephone). It's possible to change the passenger's name too, but this is even more expensive (Raynair charge 100 GBP, Wizzair 34 GBP).
10) Cheap airlines don't provide any free catering on-board. There is usually a selection of snacks, sandwiches and drinks available, but these cost a lot. If you are serious about saving money, bring a packed lunch, although you won't be able to take drinks through security: buy them in shops on the airside, as even at their inflated prices they will be cheaper than anything bought on the plane.
11) Many cheap flights leave or arrive very early in the morning or late in the evening. Public transport, especially if travelling from further away, might be limited, non-existent or expensive. In any situation, factor in the costs of travel and car park if used into your total price estimate.
12) Car parking can be very expensive - you might end up paying more for parking than for the flights. Think twice about driving, and if you need to, check off-site parking. Holiday Extras have the best deals. Some airport hotels offer free parking for two weeks if you stay a night with them, which you might want to do anyway.
13) There might be discounted public transport deals available. On some Ryanair flights to Stansted tickets for coaches to London town centre are sold, while anybody travelling by train from Prestwick will get half price ticket to the whole of Scotland.
14) Check in for cheap flights closes 30 or 40 minutes before the flight - it really does - and you will not be called for or allowed to check in if you arrive late.