Weather it’s regarded as expensive is subjective & based on what other action you can take to successfully repair what you already have, how long it will last, what sort of porch you want & on your financial circumstances. A decent, internally plastered porch with a damp proof membrane in the floor, cavity walls, lighting, a decent roof, good insulation, double glazed windows & a security door will help keep your house warm, give better security & allow a dry environment in which you can store coats, shoes, boots or whatever takes your fancy. No offence but what you currently have sounds little more than a damp, draughty timber lean-too, well past it’s “sell by” date; hardly in the same league. Rather than spend £300 on a new roof which, from what you say, may not even cure the problem you may be better to put the money towards starting again with a new structure, based on what you can afford. My porch cost £1,400 but was built to a high spec. & is under an existing, external pitched roof so I didn’t have the cost of a new roof. A pitched roof may have added another £600 but I’m purely guessing. It depends on what you want from the porch & the build spec. the only way you’ll get an accurate cost is to get some quotes in.
If you do decide to knock it down & start again, the floor area must not exceed 3 sq/m externally (10ft x 5ft is approximately 4.5 sq/m), it must not be within 2m of any boundary or over 3m high; exceed any of these & you will require Planning permission. There are other requirements (see my previous link) it must meet to ensure it complies with Building Regulations exemption.