What to Look for in a Gas Grill between $250 and $500
In this price range I really recommend staying away from any gas grill that has a lot of stainless steel. Stainless steel used on these gas grills is almost always low grade, 430 stainless. This kind of stainless steel holds a magnet so that's one way you can easily test for it. This type of metal is prone to discoloring, staining, and rusting. This means that these grills look great on the showroom floor but have trouble retaining their appearance. I recommend looking for porcelain coated steel grills.
You are going to find that these grills are much larger than the less expensive units. Don't be tempted to buy the biggest grill if you don't need that much space. I recommend grills with a primary cooking space of 300 to 350 square inches and a BTU rating around 28,000 to 36,000. While some of these grills have as many as 5 burners to heat spaces over 400 square inches the smaller grills only need 3 burners. Most of these grills have side burners and if that is something you will use (which most people don't) get it. Otherwise there is no need to pay for it. If you do a lot of rotisserie cooking (or plan to) the rear mounted rotisserie burners are nice but the ceramic type burners tend to break easily so I recommend rotisserie burners that are a standard gas port burner. Don't buy more, either in size or in feature, than you need.
When shopping for one of these gas grill don't be afraid to dig down inside to see how it is put together. It is a good idea to remove the cooking grates on the showroom floor and take a look at the burners. The burners in almost all grills in this price range are stainless steel, tubular burners. This is when a magnet will really come in handy. If the magnet sticks to the burners they are low grade stainless steel. Avoid these grills. If a burner is going to last more than a few seasons it should be heavy duty, 304 stainless steel. If the burner feels lightweight don't buy the grill.
What to Expect from a Gas Grill between $250 and $500
Gas Grills in this price range should offer you high, even heating and most of the name brand grills do. You should also get some good versatility out of one of these grills. While you will find that some shortcuts have been taken with some of the grills in this price range you should expect at least 5 years good service out of a grill that costs between $250USD and $500USD. That lifespan can easily be increased to 10 years with care and the occasional replacement part.
Since most grills come with warranties and since the brand name manufacturers offer good warranties and tend to actually honor them it is important that you register your grill and are prepared to take advantage of failed parts. Good customer support is something you should expect in this price range and if the maker of your grill doesn't offer it, you don't want their grill.
How to Care for a Gas Grill between $250 and $500
Like more gas grills it is probably going to be the internal components that fail first, particularly the igniter and the burners. Igniter failure is a Common Grill Problem and can usually be fixed easily or simply bypassed by using one of those long lighters. Burners need to be regularly cleaned to clean off build-up on them and to keep them working correctly. Get down to the burner and gently brush off any debris. Make sure that the ports (the little holes along the burner) are clear.
To protect the burners you can coat them in cooking oil (with a high Smoke Point. This will help, but oil breaks down at high temperatures and looses its ability to keep the burners from rusting. The best part of doing this regularly is that it forces you to get in your grill and clean it out. This will do more to add years to the life of this grill.