How do you measure savings for new projects?

I'm interested to hear how you measure savings for new projects; I have two scenarios and would be keen to hear if you've seen or used either of these methods, and if/when you would use either or both. 1: You're procuring a new piece of software which you have a budget for of £100k, you have 2 quotes; £100k and £25k and both suppliers can meet the specification. You negotiate a 20% discount for both offers, meaning your final quotes are £80k and £20k. Both are under budget, but should you choose the £80k supplier, would you claim a £20k savings, against £5k for the £20k? 2: Taking the above scenario, could you measure savings as difference against budget? i.e. The provider at £80k would represent a £20k saving, opposed to a £80k saving should you choose the supplier who can provide it for £20k. I'm sure there are other ways that you could also measure the savings so would be very keen to hear other options too.