Hello:
I believe an accurate budget means having more money than you THINK you might need!
You don’t give many details…
A lot depends on where in Spain you will be living: out in the sticks or somewhere on the southern coast, or in a large city. I am assuming you are buying a property, or…..are you renting?
At the moment it is a good idea to rent first, see how you like it and then buy in a year’s time, when you have had time to decide what you like and where you like.
If you own a property you have to factor in monthly maintenance costs, (rates/council tax, condominium service charges, (known here as cuotas de comunidad), electricity, water, contingencies, possible repairs, insurance(health insurance and so on), transport, the possibility of going out to dine now and then, short breaks to other parts of the country or to the rest of Europe…..
My own view is that you would need at least 1.500 euro per month, probably closer to ?2000.
All the best
Campana