Obviously you want CC as anything else is done more cheaply by Crusader Squads, the first problem is whether to take Lightning Claws or Powerfist. You want Terminator Honors for either one, and the Powerfist can hurt vehicles so without it you'd need Krak Grenades, but which one is better? With Lightning Claws (on the charge) you have 4 attacks, 2 hits, 1.77 wounds, so 1-2 kills (this is against MEq). With a Powerfist you would get 3 attacks, 1.5 hits, 1.25 wounds, so 1 kill usually. The Lightning Claws take advantage of the better Initiative when charging, but the Powerfist can Insta-kill. But you could take a Storm Shield with the Powerfist for the cost difference. Personally, I'd take the Powerfist for the vehicular advantages. Besides to make up for the cost difference you could simply not take Terminator Honors of the Lightning Claws, which means only about 1 kill so you lose that advantage.
I'd take two Storm Shields to absorb Power hits if you have the points, though I would not put them on the guys with the Powerfist/Power Weapon in case they don't make their save. Give them Frag Grenades, Furious Charge, and put them in a Rhino, a Land Raider Crusader if you have the points. Join your Emperor's Champion to them so he can use his Black Sword to full effect. As to how many, this is very much up to your point limit, but since we are trying to save points rather than having Terminators we'll go with 5.
So here is the unit:
Sword Brethren Squad (5): BP/CCW, Frag Grenades, 4 Terminator Honors, 2 Storm Shields, Powerfist, Power Weapon, Furious Charge, -190
Rhino: Extra Armor, Smoke Launchers, -58
Are they cheaper than Terminators? At a total of 248 compared to the minimum 480 that Terminators with Furious Charge In a Land Raider Crusader would be, yes. Are they more effective than a Crusader Squad would be? An equivalent unit would be 9 Initiates with Frag Grenades and a Powerfist -168 plus transport (we could exchange 4 of the Initiates for Neophytes but that would bring the cost down too much and we are trying to fit in a rhino).
The Sword Brethren get, on average, about 1/3 more kills on the charge and 1/6 more kills in standard combat. Sword Brethren have 2 3++ saves but the Crusader Squad has 9 wounds to clean out rather than 5. Plus the Crusader Squad can put out reliably 1 wound in shooting while the Sword Brethren have 3 Bolt Pistols so, 1 if they are really lucky. My calculations are all against MEq and I estimate the Crusaders to gain an advantage against GEq as well as TEq. Plus the Crusader Squad is Scoring so it can do more than contest objectives. It is less reliable against Vehicles but you can snatch a Melta Gun for +10 points (and you're still cheaper than the Sword Brethren). So again, are Sword Brethren better all around than Crusader Squads? No. They have slight advantages but are not worth enough for their points.
Compare once more to Terminators (not Assault Terminators) who do not fire their weapons and have Tank Hunter instead (5 members). They will most likely kill any Vehicle they come in range of, so that's a small advantage. On the Charge they do 3 more kills than the Sword Brethren, 6 1/3 more normally. In Shooting they have 5 Storm Bolters so no need to worry about that. They are so much better and, if you give them a Drop Pod instead of a Land Raider they work out to 3 points less than the Sword Brethren with their transport. You could, of course, give the Sword Brethren a Drop Pod instead of a Rhino, and save a whopping 20 points. For 17 extra points, I would take 5 Terminators in a Drop Pod. And you don't even have to take a Drop Pod, you can Teleport them in. To add onto all of this the Terminators have a 2+/5+ save instead of the Sword Brethren's 3+ and 3++ saves. So they're more Survivable as well.
Now you could improve the Sword Brethren Squad with more members, which would increase the price and still never reach the effectiveness of the Terminators.
To put a lighter note on them, Marshal Learoth has just designed a list using Razorbacks with PotMS. For an elite choice that fits with this, Sword Brethren are viable. But what Marshal Learoth took instead of what I was thinking was 3 Vindicators which are a far, far better choice than the 2 extra units I was thinking of.
I admit, this analysis does not cover absolutely every build. You could Infiltrate the Sword Brethren with a Multi Melta or a Melta Gun and get a few shots off, or you could spend the points more efficiently on something that isn't a suicide unit. Frankly, the best way to use Sword Brethren is done far better for the same price by Terminators and done more cheaply by Crusader Squads. Sword Brethren just aren't worth it.