Cost Of 500
Pits by Honda $750.... to possibly half that with others (Mag 7, Baja Pits)
Score entry aprox $800 plus a annual $50 for score card.
Depending on how you exist for accomadation, gas, food $100/day to $250/day. Theres the whole range in Ensenada. If you're using a campground/ locals paddock/ or hotel in outlying towns it tends to be cheaper/ simpler in all respects (food/accom).
Bike. You can ride it stock as long as you have got 50/60 mile tank capability for race. Prerunning you might want a larger tank ( $200). If you're going for the basics invest in suspension ($850- best money can buy), damper (a lot but worth it), footpegs ($125), shark fin, Handle bars......and all the rest.
For 500 hopefully you won't need lights. You may want a plan B which might involve a helmet light ($250- $400, if you've already upgraded your stock bulb) just to get you last miles. Otherwise you're looking at a stator rewind and a single/ double 8" or Diablo from Baja Designs. ($125 + $200- $1200)
Vehicle insurance. For car/truck/van- $10/day.
Cost would depend a lot on amount of prerunning ie tires, odd tube, few levers, filters and oil. A rear for every 500 miles prerun, a front. A chain, set of steel sprockets.
For race you'll need fresh tires all around, oil,filters, chain, sprocket etc
If you're doing a LOT of prerunning I wouldn't suggest riding same bike for the race. It'll be pretty beat.
The 500 is a good race and logistically reasonably easy. It can be done for relatively cheaply. Not everyone is driving a Trophy Truck nor is every bike prepped to Honda race team standards.
On the day - enjoy. There won't be any amount of money that can pay for that experience!!! A.