I don't know really. The wall above would need supporting while the existing lintels are taken out, you'd need a new lintel plus a window then plastering etc. The lintel won't be expensive.
I'm an technician, we don't really get invloved in pricing but i would have thought around £1k for the supply and fit of the new lintel then fitting the new window on top. You'd get change from £2k all in.
I'd just get a local builder to come round and give you a quote.