I think a lot depends on what you currently spend each month (and whether you want to stick to similar level of outgoings, or would be happy to double, treble, or quadruple your spend).
Yes, if you currently spend £5-£10 a month {and get some bolt-on or Freebee to give you X minutes and Y texts for general use} then it could be significantly more if you choose to go for some top-of the line smartphone, which every network website and salesperson will try to sell you.
First I'd say "Which network are you using" because they may have some deals available, especially if you have been a loyal customer for some years.
Second, I'd ask whether you would be prepared to spend say £150, £300, or £450 right now. That could easily be a "NO!!" at the higher levels, but if you wanted to go for some of the tablet or similar "big screen" options, it is sometimes worth paying a lump sum rather than the "spread over 24 months" costs which many phones and tablets come with (particularly at higher end, but not exclusively).
Third, I'd ask whether you'd be willing to "dip your toe" with a refurbished phone, because sometimes there are bargains to be had, or do you have work colleagues / family / close friends who've looked down their nose at your current phone and would not want a slightly out of date phone, if only because some might still make snide remarks about them having had one of those in 2012 or they replaced theirs in January 2013...
One of the lower cost (but perhaps mid-range in facilities) is the Motorola Moto G at prices from around £100-£150 depending on whether it is the 16 GB or 32 GB model and whether you've found a bargain or are paying top dollar... HotUKdeals.com LINK