Bad news is that exhausts for these motors aren't cheap!
I can only presume your exhaust is the same as my '99 2.5 as i don't know if facelifts added anything to the pipes!
I recently looked into exhausts a lot (hence the length of this post!) and found varying prices for a 'cat-back system' (which is what you want - 2 centres and a rear)
Costs depend if you want to fit it yourself, or have a mate who can, or whether you'll pay a 'professional'. Also, whether you want a 2.5 rear box (single pipe) or a nice 3.0 rear box (two pipes) for an extra £40ish. they look better i think!
The best system for you depends on mileage. If, like me, you do a lot of short journeys then consider a bespoke stainless system from the likes of Powerflow or Longlife. Then it won't rot, and should cost £300-£400. You can pick the look and the sound!
However, some would argue that steel ones sound better, and shiny isn't always good.....
Of these, Vauxhall make the best system for the car. The performance of it can only be bettered by messing with the airflow of manifolds i believe! Direct from them, the retail cost is near £400inc VAT and then you or them or someone has to fit it. Through trade club tho, and the price is more like £260incVAT (plus fitting)
I've just replaced the centre boxes on mine which were 9 year old Vx ones that'd done 85k

If you search the forum, you'll find similar ages and much more mileage can be had from them!
The last option is the budget system that'll not last anywhere near as long and isn't actually much cheaper. But available if you really want. I found them (Bosal or Timax), to also be around £250 from centres.
But you can get just the parts to fit it yourself from places like buypartsby
http://www.buypartsby.co.uk/exhausts_display.php?txtmodel=Omega&Submit=Next&txtmake=VAUXHALLor leisureshack
http://(http://www.leisureshack.co.uk/osb/showitem.cfm/Category/Omega_15426These work out at £140-£165 inc rubbers, bolts and delivery.
Oh the last option? Buy secondhand and get through an MOT with it and hope it'll last for at least twelve months. Personally, after spending a wedge on everything else on the car, the stainless system is on hold and £50 got me a 3 litre system from my new friend. Thanks Timbuk!

Hope that info is of some use!