Hi, I am in the UK. But it's kind of hard to answer your question about vet costs because it does depend on what needs to be done. I am recalling the costs I used to pay at my vet's but that was over 2 years ago now (my dog passed away over 2 years ago.)
Although vet's charges do vary slightly, a simple check-over with NO treatment/tests and NO medication would cost less than £20.
But how often does that happen? There will always be some kinds of treatments or tests if what is the matter with your dog isn't obvious and very mild indeed. Even then -the least you would go home with would be a bottle of stomach medicine or a worm pill.
So it is very hard to estimate what it might cost you.
However my serious advice is get your dog to a vet. You have no idea what is wrong, and for all we know it COULD be something serious, which you would deeply regret if you didn't act to help him.
I mean....for goodness sake -take your computer/watch/gold ring or whatever to a pawn shop or something to get the cash if you are very strapped for cash
I'm in the USA so I have no idea what UK vet costs are.
If the scent sacs need to be "expressed" I think that's what it is called, a matter of applying pressure to either side, and it can be an unpleasant experience, but if needed, well needed. A groomer may do this for a lower cost than a vet, or in the US it may be possible to the a vet tech to do the work without a vet being involved.
You didn't mention the age of the dog, but it seems you need to find some way to get a check-up from a veterinarian. You should be able to get some help from a local vet on what arrangements they can make for payment or a reference to call for help from a shelter of some type.