Upgrading a Warden/Errant to a Crusader is 55pts. 5 more points and you get a rap battle cannon.
I tend to look at it as upgrading a Errant/Paladin to get an avenger gat for 55pts.

The alternative is to upgrade a Warden/Errant/paladin with a Storm Spear Rockets for 40pts.

Lets compare the avenger gat to the stormspear rockets.

avenger gat is 36" S6 ap3 heavy12 rending while the Stormspear Rockets are 48" S8 ap3 heavy3 but costs 15pts less

avenger gat S6+rend

av11 glance 33% pen 16% x 12 shots = 4 glance 2 pens
av12 glance 16% pen 16% (due to rending d3 of 1s = 13) x12 shots = 2 pen avg
av13 glance 16% (rending d3 1s) pen 5% (rend d3 of 2s=14) x12 shots= 2 glance avg and .6 pen avg
av14 glance 5% (6s/2s=14) pen 2.5% (6s/3s) x12 shots= .6 glance avg and .3 pen avg
T4 (terminators and marines) 83% wounding x12 shots= 10 wound avg and 2 ap2 on termi
T8 (freakin Wraith Knight): 16% wounding x 12 shots = 4 wound avg (suck on it WK you get no armor from rending ap2 so take your invul and fnp which you will probably save anyways :( )

EDIT: forgot to account for BS4 = 66% hits of the 12 shots. So the ending avg should be much lower
replace 12 shots with 8 hits then calculate %.

Stormspear Rockets S8
av11 glance 66% pen 50% x3 shots = 2 glance avg and 1.5 pen avg
av12 glance 50% pen 33% x3 shots = 1.5 glance avg and 1 pen avg
av13 glance 33% pen 16% x3 shots= 1 glance avg and .50 pen avg
av14 glance 16% pen 0% x3 shots= .5 glance avg and 0 pen
T8 wounding 50% x3 shots = 1.5 wound avg still ap3 atleast.

For 15pts more you can take an avenger gat over the Stormspear rockets. However 2 avenger gats =110pts. 3 stromspear = 120 pts. Are 3 Stormspear Rockets better than 2 avenger gats?

2 avenger gats for 24 shots for 110pts
av11: 8 glance avg and 4 pens
av12: 4 pens avg
av13: 4 glance avg and 1.2 pen avg
av14: 1.2 glance avg and .6 pen avg
t8: 8 wound avg

3 StormsStormspear Rockets of 9 shots for 120pts
av11: 6 glance avg and 4.5 pen avg
av12: 4.5 glances avg and 4 pen avg
av13: 4 glances avg and 1.5 pen avg
av14: 1.5 glance avg and 0 pen avg
T8: 4.5 wound avg

I dont know how to math hammer range comparison of 36" vs 48"

Now lets compare Thermal Cannon vs Rapid Battle Cannon

Thermal Cannon S9 ap1 ordinance 1
av11: .83 glance .66 pen
av12: .66glance .50 pen
av13: .50 glance .33 pen
av14: .33 glance .16 pen
T7: .83 wounds
T8: .66 wounds
T9: .50 wounds
T10: .33 wounds

Rapid battle cannon s8 ap3 ordinance 2
av11: 66% glance and 50% pen x 2 shots = 1.33 glance avg and 1 pen
av12: 50% glance and 33% pen x 2 shots = 1 glance avg and .66 pen avg
av13: 33% glance and 16% pen x 2 shots = .66 glance avg and .33 pen avg
av14: 16% glance and 0% pen x 2 shots = .33 glance avg and 0 pen
T6: 1.66 wounds
T7: 1.33 wounds
T8: 1 wounds
T9: .66 wounds
T10: .33 wounds
Edit: forgot to account ordinance allowing roll 2 d6 and take highest to penetrate.

However Termal Cannon trumps Rapid Battle cannon when it comes to killing terminators
say 5 terminators are under the blast. 5 x .83 = 4.15 wounds no 2+ armor means 1.38 invuls saved as the remaining 2.76 fail.
Compared to Rapid battle cannon 5 termis x 1.66 wounding= 8.33 wounds but 2+ armor saves 6.94 terminators 1.36 fail.

We can also compare Crusader melee vs having a chain reaper sword.

Crusader A3 S10 ap2

T8: 83% wounding x 3 attacks =2.5 wounds
T9: 66% wounding x 3 attacks = 2 wounds
T10: 50% wounding x 3 attacks = 1 wounds
av11: 83% pen x3 attacks = 2.5 pen
av12: 83% glance 66% pen x3 atk= 2.5 glance avg and 2 pen avg
av13: 66% glance and 50% pen x3atk = 2 glance avg and 1.5 pen avg
av14: 50% glance and 33% pen x3 atk = 1.5 glance 1 pen

Chain reaper sword S*D
T#: 16% nothing, 66% d3 wounds, and 16% RFG
av#: same thing as above but with hp
Super heavy Vehicle: 16% Nothing, 66% d3 hp loss, 16% 6+d6 hp loss.
(how do you compute avg d3 hp loss and 6+d6 avg hp loss?)

I am not sure how to compare point cost efficiency.