Try this...

select r2p.offer_pricing_gen_seq, sum(cu.contractual_consumption)/1000,
r2p.ramp_hub, r2p.ramp_basis, r2p.spot_hub, r2p.spot_basis, r2p.risk_premium, sub.risk_premium
from r2_price R2P, contractual_usage cu,
(select r2.risk_premium from r2_price r2 where r2.product_id_gen_seq = 37 and r2.term_gen_seq = R2P.term_gen_seq and r2.offer_pricing_gen_Seq = R2P.offer_pricing_gen_seq and r2.pricing_parm_gen_seq = R2P.pricing_parm_gen_seq) sub
where r2p.offer_pricing_gen_seq = 9155 and r2p.accepted_ind = 'Y' and cu.offer_pricing_gen_seq = r2p.offer_pricing_gen_seq
group by r2p.offer_pricing_gen_seq, r2p.ramp_hub, r2p.ramp_basis, r2p.spot_hub, r2p.spot_basis, r2p.risk_premium, sub.risk_premium

I don't think it will receive the right data, but the query should run.