There are not that many extra features in phono amps so yes usually the extra money is just for increased sound quality, increased build quality and premium materials.
How much is a reasonable amount to spend? Well depends on how good your currently system is as that will be the bottle neck and limiting factor in the system.
Assuming the current system is not limiting the performance. I think most people will easily hear improved sound quality as the budget increases up to £200/300 price point and then it starts to tail off up to around £500-700 price point where a lot of people don’t see much improvement in sound quality.
If your system cost a £300-400 a couple years back its probably an OK system I think you’d be justified in spending around ~£50 on the phono amp to get a good reasonable sound.
There are many Internet only sellers now selling direct to consumer through a website or through Amazon or eBay that produce value for money sound quality. I’m not up to date with the brands and models so can’t recommend any but a search on the web should give you some suggestions.
That or have a look at Richer Sounds and see what they have in their entry level selection.
Schiit Modi. Thank me later.
Isn’t that a DAC? The OP wants a phono amp? Although the Schiit brand appears to be getting some good reviews for some of their products and are at the budget end of the spectrum so is well worth a look to see if they do a phono amp.