The engine definately doesn't have to be pulled to change the cam. Some people drop the cradle to pull the cam out instead. Personally, I drill a hole in the fenderwell and pull the cam out without pulling the engine or dropping the cradle. Then when I put the new cam in, I rivet on a piece of aluminum over the hole i drilled and paint it with undercoating and you can barely even tell it was done even if you're looking for it. $400 is a decent price to have someone do it and I can't really see you getting it installed by someone else for much cheaper, but if you are pretty well mechanically inclined you can get a few different write ups on clubgp for it then you should have no problem doing it yourself in a weekend.
Still for someone schooled in doing it, it would be a 6 hr job at least.
For someone that has done cam swaps before and does work on cars regularly 6hrs. would be a reasonable time to change the cam in, but it is very possible to do it much quicker. The president of the MI GP club can do a cam swap in ~2hrs, and he pulls the engine when he does it, but he'* also done a LOT of them. I've only done two (in GPs) and I can do one in right around 6hrs.
1990 Maroon Grand Prix STE McLaren 3.1 Intercooled Turbo, 14.695 @ 94.49 on 12psi before cam/heads/etc.
1990 Bright Red Grand Prix McLaren 3.1 Intercooled Turbo, 14.819 @ 92.72, 189whp
1999 Grand Prix GT 9.75:1|K&N|160 tstat|No Ubend/Cat/Res|UD Pulleys|Ported M90/TB/Heads|XPZ|2.55"|Pacesetters|Stage 2 I/C. 12.442 @ 112.78
2000 Blue Bonneville SSEi Supercharged Gutted Airbox|K&N|3.4"|1.9'*|WP Pulley|Custom DHP|180*stat|41932'*|No Cat. 14.681 @ 94.80
2002 Dark Cherry Grand Prix GTP Supercharged & Intercooled
12.084 @ 114.38 w/1.743 60', 315.6whp, Fastest Stock Cam M90 GTP!
NOW XP CAMMED, 340.6whp with XP, stock heads, Gen 3 M90, 11.984 @ 115.84 w/1.750 60'!
Most HP Stock Heads Gen 3 M90 GTP!
2004 Red Monte SS Supercharged K&N|3.4 Pulley|1.9'*|180*stat|41919 plugs|3" dp|No cat/ubend|DHP|Shift Kit. 14.2 @ 98
GOT BOOST??? Friends dont let friends drive stock!