Depends what you want. I got a tyre inflation compressor that'll inflate a 275/70-16 tyre from almost flat to 38psi in 2-3 mins for under £30 and it's a decent quality, metal bodied pump, not one of those cheap 'disposable' plastic hobbies either. Most people don't need an onboard compressor with built in tank that can inflate 4 tyres from flat as quick as a garage hose so saying you have to spend £300 is rubbish.
it's not rubbish!
I'm saying don't believe the adds that tell you a 30 quid compressor will infalte to 100psi, because thats not whats important.
I also said a couple of hundred quid for a compressor worth having, not 300, thats the cost of my set up. I'm not denying you can get cheaper, just that IMHO and experience, they aren't worth having. They are unreliable and inadequate for reinflating four tyres, and will probably let you down just when you need them most! Just because it's not the same as your opinion doesn't mean it's rubbish!
As you say, it depends what you want it for, now try doing all four tyres in a row with your compressor, then try doing it in 40 degrees in a sand storm!! It was while doing this, when both my truckair compressors that worked fine in the UK when I tested them blowing up a single tyre, packed up within a minute of use that I decided I really needed a better air compressor! (I contemplated this while reinflating 8 tyres with a foot pump, in said sand storm at 40 degrees!!) You don't need onboard, but I needed it for the airlockers anyways so went for a bigger compressor and tank as that worked out more cost effective than getting a separate portable compressor and another one to operate the lockers (a cheap compressor will keep thermally cutting out if you try and do all four tyres).
If you don't have a reliable method of reinflating tyres it discourages you from deflating them when you need to, but, as you say, it depends what your going to use it for. I spent several years researching the options before deciding what to go for and all the options on my final list were going to be over £200, whether freestanding or onboard because in the environments I visit, a 30 quid halfords special aint going to cut it. I need it reliable and powerful enough to reinflate all four tyres quickly.
Slightly off topic, but for those looking for reliable tyre inflation, Viair are pretty good and some of the more competitively priced.
Even then one like this
http://www.mattsavage.com/acatalog/300P-Portable-Compressor-Kit-30033.html
will take over 10 minutes reinflating a full set of four 33 inch tyres from 15psi to 30psi, and thats not allowing for the duty cycle if it's overworked. That one is over £160 and for me is not really up to the job.
this is probably the smallest I would consider, partly for flow rate, but also run time.
http://www.mattsavage.com/acatalog/400P-Portable-Compressor-Kit-40043.html
The other aspect is, without an adequate flow rate, you may find it difficult to get a completely flat tyre to reseat on the bead
