I bought a Saleen replica wing off of some body kit company about 4 years ago.....absolute POS....came in chipped and cracked, had to fight with UPS over it even though the box was in perfect condition (meaning the company had done the damage to the product, not UPS). They finally replaced it for me AFTER I had to pay for it and the shipping a second time and I got another POS wing that was again in very poor condition. I ended up getting my $$ back after 3 months of fighting with them, but was still out over a $100 for shipping and ended up getting it from Saleen in the end. Point is...thought I would save a buck and ended up spending more!
Don't forget too that some kits will require more body shop work, so saving $300 on a kit and then spending an extra $400 prepping it for install doesn't save you anything either.