My paint horses seem pretty prone to dry, cracked hooves as well, the light, superficial cracks on the surface of the hoof are pretty much constant no matter what I do, however, hoof cracks like those shown do need a bit more than supplements and coatings to protect.
My paint mare had a big crack down the front of her hoof and I work with a gal who's a farrier. I was told (and this is what my farrier did) that sand granules can cause cracks like this - that if the farrier would rasp or dig out a chunk and find the problem at the coronary band, the crack would grow out. So he rasped a little ridge across the top of that crack and it's nearly grown out now - it's just a superficial little crack when the weather gets dry.
I've tried both supplements and topical remedies. The feed thru, although expensive, do help more than the topical and are a LOT less work. When you apply something to the hoof surface, you also lock OUT moisture as well as lock it in. So if you take a dry hoof, coat it with Hooflex or Rainmaker or something like that, it doesn't really make the hoof better. The best way to use topicals is to soak the foot well - if the horse will stand with it's feet in a tub or feed pan of warm water for a while each day, then you dry the hoor then apply the topical. There's a lotion by Mane & Tail called Hoofmaker that's just like hand lotion. You brush off or clean the hoof well, then apply the cream liberally to the whole sole and surface of the hoof.
I use a product called "hoof biotin" and I purchase it from Uckele. I purchase it in bulk - it runs about $150 per container and the container lasts my two horses about 6 months, so for one horse, it would probably last a year. That only equates to less than $20 a month, which is pretty cheap. You can buy it in smaller jars, but the cost is higher then as you're paying for much more packaging than supplement.
Unfortunately, about the only way to tell for sure whether what you're using is working or not is to watch it closely for a good six months - then if you're skeptical, take them off the supplement and see what happens for the next 6 months. Although small, superficial cracks aren't pretty, they are not to be considered poor health either.
I might also suggest adding some sort of Omega 3 and 5 to the horse's feed. Just as in humans, these make our hair and nails grow better and can affect dry eye and dry skin. Perhaps adding an oil would help you - I use a product by Uckele also called "cocosoya" oil - which is nothing more than unrefined coconut oil and soybean oil. It's really good for the horse. I buy it in the 5 gal container for about $50 - I feed it to 3 of mine twice a day and it lasts maybe 3 months, so for one horse, I bet such a thing would last nearly a year also. It can be purchased by the gallon for about $20, so I'd suggest purchasing a gallon to try it for a couple month, that way you have the gallon to refill from the larger container if you chose to purchase the larger one. If you are where it gets cold in winter the stuff gets pretty thick when cold and doesn't flow thru the pump well - I take my gallon in the house with me in winter but in summer it stays in the barn.
While many supplements seem expensive, you need to know how much they require and how many day's supply you're buying. Hoof supplements are some of the least expensive ones. I used to have good luck with Vitamin - Biotin by Farnam but it's been harder to come by, takes more to feed - it costs no more really than the competition though.
Finally, if you're looking to see what works best, isolate trying one thing at a time - for example if you try the supplement don't do anything with topicals at the same time - otherwise how will you know which one worked? You won't see results until the hoof grows a good amount, so the test likely will be a good 6 months to get a true idea of whether it's helped or not. The condition the horse is kept in - dry lot vs. grassy pasture with some mud, plays a role too - if the pasture is always dry and his stall has kiln dried bedding, this will suck the moisture out of the hoof (this happens to us printers who handle paper all day-it sucks the moisture out of our hands) - see if you can change it up a bit so he's in moisture some. The horse's diet may play a role too- switching his feed to something with more Omega 3 and 5 might do the trick by itself.
Good luck!