At 200pts for a full horde of 40 clanrats, which gives them +8 starting Bravery, +1 to hit, +1 to wound, and +1 Save, spending another 100pts on a clawlord for the Gnash-gnaw on their Bones! command ability (+1 attack) turns the unit stat-wise into discount basic stormvermin that happen to come in bulk and are only slightly worse at piercing armour.
If you're looking for the hard math of this, 300pts invested in a full horde of clanrats and a supporting clawlord makes each clanrat in the unit worth about 7.5 points each, which is little more than half the per-model cost of a minimum-sized (10-rat) and unsupported stormvermin unit (14 points per stormvermin) whose only advantages are a weapon rend characteristic and higher starting armour save before Clanshields is applied. Whilst it can be argued that after horde bonuses a still-unsupported full-sized unit of 40 stormvermin are objectively better quality troops regardless whose stat advantages are worth the discounted 12.5 points per model, that also happens to be a 500pt unit whose stats are counter-balanced by having half the wounds and two-thirds the potential attack output of 2 full hordes of clanrats with shared clawlord support for the same pricetag.
With that said, if you're still intent on playing a Clans Verminus horde army, I highly recommend you pay the 180 points for a claw-horde warscroll battalion. It requires 1 clawlord, 1-3 stormvermin units, and 2-9 clanrat units, and, on top of the usual benefits of using a battalion warscroll, it allows you to make the clawlord's command ability even more cost effective as it will now affect all in-battalion units wholly within 13" instead of just one Clans Verminus unit within range.
