Welcome to the forum and sorry to hear about your wee one - you will find lots of help, advice and support on here as sadly many of us have either had or still have dogs with SM.
I'm presuming you are in the UK? according to the British National Formulary Gabapentin is only available in the following sizes
100 mg, net price 100-cap pack = £3.57; 300 mg, 100-cap pack = £8.83; 400 mg, 100-cap pack = £5.53 [the prices quoted are net of VAT and are usually correct, they show the huge markup there is on drugs, but remember that veterinary practices rely on income from drugs to support the practice, otherwise fees for consultations etc would have to rise considerably. They also have a legal responsibility for the dog for medication they prescribe.]
I suspect that the price you are paying is due to it being specially compounded to obtain this dose. You can compound it yourself, you can obtain empty capsules from a pharmacy or even on E-bay now! You can tip the contents of a 100mg capsule onto a piece of folded paper, and then carefully tip half into a new capsule and half into an old one, and reseal. It's a bit fiddly but doesn't take too long to do. [At one point one of my Cavaliers was on an epilepsy drug and there was a manufacturing problem, so we could only obtain very high dose capsules - I had to split each capsule into 6 doses :yikes] Of course you will need to get the 100mg capsules or a prescription from your vet for which they may charge, ask them to put repeats on it too, and you can obtain the drug from pharmacies - I think Boots the chemist is one of the cheapest right now. Your vet may not be too happy about you splitting capsules, as it's impossible to be totally accurate, but if you are very careful it balances out over the course of a day or so.
I think Gabapentin is available in a liquid form in other countries, but not the UK - it is not recommended as it is very irritating to the gut lining, another reason why this drug should only be given in sealed capsules.
BTW anyone can register to access the BNF
http://bnf.org/bnf/bnf/current/129255.htm#_129255
Have you seen a neurologist? Really they are the best people to see as they have experience with SM and are able to do a full consultation. Also have a look at
http://sm.cavaliertalk.com/ and
http://www.veterinary-neurologist.co.uk/ [from where you can download a treatment alogirthm - under Syringomyelia, downloadable PDFs] It is usually recommended for your dog to be given one of the CSF reducing drugs, Clare Rusbrdige recommends Zitac [cimetidine as being the gentlest for long term treatment]
Hope this helps