Bilsteins are top-of-the-line units and priced as such. The OE Boge (now Sachs) units are very good, and sold by many of the banner advertisers here. Check their prices, and consider that a repair shop could probably get them cheaper than you can. That gives you an idea of their labor charge, and thus, whether you want to try the DIY route. I'd be a bit wary of the mufflers-and-brakes-shop route, as they may not be familiar with Volvo idiosyncracies, and tend to use the cheapest/cheesiest parts they can get. I'm not terribly familiar with front end work, but you may need a wheel alignment after this job, and if so, make sure it's included in any price quote, or factored into your DIY cost calculation.