If you are flexible about travel times, before you book any flights with budget airlines such as Ryanair or Easyjet, plug the details into Airhint (www.airhint.com) and see if you could do better by changing your flight. If you book the return sector of your flight separately, you'll often pay the same figure in Euros as Pounds, which will save you around 10%-15%, depending on the exchange rate on the day, and whether your payment card punishes you for making a non-sterling transaction.
If you can't get a direct flight to your destination on a particular day, then look up one of the flight comparison websites (my favourite is Skyscanner - www.skyscanner.ie), plug in the details and you will see alternative ways of getting to your destination. You can enter alternative airport destinations, if for example, Malaga, Granada, Murcia or Alicante were acceptable. Remember: if you are warned about "self-check in" or words to that effect, it means you must check out all your stuff at the intermediate airport and check in again for your second flight. This is a bother when you've several people in tow, and a big stress if the first flight runs late. However, these tend to be the cheapest of the indirect flights.
Getting to Roquetas is most convenient (or rather least inconvenient!) from Malaga. Get the metro (called Cercanias) from the airport to the main bus and rail station (it's called Maria Zambrano, after a local poet. This is frequently shortened to MZ). There are several buses a day to Almería (mostly just 1- or 2-stops), and from Almería take the local bus to Roquetas. Alsa is the biggest operator (www.alsa.es - click "en" or the Union Jack for instructions in English). If you contact Blablacar (www.blablacar.es) they'll put you in touch if there is a car and driver going the same way as you. Expect to pay a contribution towards diesel which compares with the travel cost by bus, but you'll receive something approaching a door-to-door taxi service for a fraction of the price. There is no convenient train service from Malaga to Almería.
There are bus services from Murcia and Alicante to Almería, but none of the ones that I've seen are as convenient as those from Malaga. For more information on bus services from these places look up rome2rio (www.rome2rio.com) which will tell you the options that are available at your time of travel. There is muttering about connecting Alicante airport to Alicante by rail (the existing line to Elche-Elx passes only some 2 km from the airport), and I heard that the planners are seriously looking at this. At the moment, Alicante airport is surprisingly badly served by buses to Alicante. (In fact the only similar-sized airport with such lousy bus connections is Faro in Portugal!)