I am looking for a cost friendly joint supplement. I was considering getting simply equine's structural boost, as that is what they recommended for him. I wanted to see if anyone else had any suggestions. At the moment I need to stay around $30 a month. I did have him on acti-flex before. 

Adequan series runs about 40/dose. You know your horse is getting 100% of the joint medication and it works amazing. Any oral product has a 2-6% absorbtion rate. I would rather spend 40 and have my horse get 100%. 

These are the ingredients for the SE you mentioned:

Bladderwrak, Devil’s Claw Root, Dulse Leaf, Frankincense, Hawthorn, Horsetail Herb, Irish Moss Herb, Kelp, Peppermint Leaf, Plantain Leaf, Scullcap Herb, White Oak Bark, Yucca Root.

Nothing I would rely on to help my horses joints in that. Adequan is a good one to do to make sure your horse is getting 100% of what you're paying for. I have my horses on the feed through Actistatin as the absorption rate is higher than most feed throughs but I still do Adequan and Legend on top of that. 

The loading dose is the issue for me.

If your horse has a chronic lameness, you are best to consult your vet about injectable support. If you are looking for ongoing support for horses that are not suffering a chronic lameness, then there are a number of well made feed throughs that can support joint health on a daily basis. Platinum CJ and Four Flex are two that are joint support only and well made. Renew Gold Senior covers it too without additional mixing and within your budget for the additional support. 

How much value you will get from any feed through joint product really depends on what the horses needs are and if you are trying to treat a lameness. If an actual lameness is the issue, injectables are a better answer. I do not like to inject horses unless I absolutley have to, so if you are looking for long term joint support for a sound performance horse, a well made feed through can provide daily added protection for hard used joints.  

I would suggest saving your money (no joint supplement at all) and then get him on Adequan when you can and maintain that.

My vet suggests to me, for my horses, to give the loading dose, and then once a month after that to maintain.

If you are going to spend your money wisely, I would spend it on Adequan.

Summit Injections.  Loading dose is $132.00.  Then 1 to 2 doses a month depending on how much you haul.  

I have heard some stuff about Summit but it hasn't been out very long has it?

You could get by on glucosamine injections until you can do the adequan. It's super affordable, even the loading dose is only around $50 and then it's a few dollars per dose after.  And research is showing lately its extremely close to providing the same results as adequan. Or give up some entry money and skip a couple events to pay for the adequan. The needs of the horse always come before any entries. If you have care credit and your vet takes it, buy the loading dose from the vet on the care credit and pay it off as you can. 

Other injectables to look into would be Polyglycan, Summit, Pentosan. I'd consult your vet about what's best for your horse and go with an injectable vs a feed they myself.  I quit doing feed they joint support years ago bc they're not getting a whole lot of it.