As always, the answer is, it depends.
What Peeps said: What are you planning on cooking? How much cooking real estate do you need? Is this just going to be a smoker, or are you expecting to grill as well? Are you determined to get something new, or is used an option? Or, could you consider doing a full DIY project or a modest DIY project??? Do you want to tend a fire or are you looking for something that's more set and forget? Are you just starting out smoking or do you have experience using a particular cooker??
Am assuming based on the question that you are not necessarily someone with a great deal of experience with Q. My apologies if that is an incorrect assumption. Based on past experience, are you someone that once you get into something, you REALLY get into it? Am asking because if you ARE someone that has this tendency, that may influence the decision. Would suck to get into something with a $500 budget and then 6 months later figuring out that you really need something closer to $3,000 to meet your new needs.
At that price point, there are several ways to go: UDS as a standalone project or use the Big Poppa kit to help move the project along. Could go for a ceramic cooker:[url="http://www.amazon.com/gp/product/B00GJEPTJS/ref=pd_lpo_sbs_dp_ss_1?pf_rd_p=1535523722&pf_rd_s=lpo-top-stripe-1&pf_rd_t=201&pf_rd_i=B00IIWDQSU&pf_rd_m=ATVPDKIKX0DER&pf_rd_r=1TN976CA9X2HVGM007MY
a modest offset e.g.,
http://www.amazon.com/Char-Broil-13201747-05-Longhorn-Offset-Smoker/dp/B00CP5G3WW/ref=pd_rhf_se_p_img_4
something like a Traeger
[url="http://www.amazon.com/Traeger-Junior-Digital-Thermostat-Controller/dp/B00DUQHAX6/ref=sr_1_1?s=lawn-garden&ie=UTF8&qid=1408989434&sr=1-1&keywords=traeger
WSM http://www.amazon.com/Weber-731001-Smokey-Mountain-Charcoal/dp/B001I8ZTJA/ref=sr_1_2?ie=UTF8&qid=1408989990&sr=8-2&keywords=weber+smokey+mountain
I'm sure there are other options in that price range (e.g., things like electric smokers, propane, etc).
EDIT: Sorry, links not embedding properly (user error!)... just clip and go to another browser window.
Bruce