The only 'cheap' way to provide UVB light for dragons, is to take them outside in natural sunlight, if the weather in your area permits it (should be at least 70F outside without or little wind, above 75F is better). About an hour of natural sunlight a day for about 3-4 days a week will ensure your dragon has all the UVB he needs.
The cheapest bulbs to get that will still provide sufficient levels of UVB for your dragon, would be repti-glo or repti-sun (two different brands) 5.0. Depending on the length (should be at or near the total length of the cage, so depending on the age of your dragon, this could be anywhere between 18" to 48" long (smaller ones are fine for 20 gal tanks, till your dragon is around 14-15" then he will need to be moved to the adult sized cage, which should be a minimum of 2'x4' of floor space and between 16-24" of height. Thus, in a cage this big, a 3-4' long uvb light will be needed. UVB light tubes vary in cost depending on their size and rating. The higher the rating the more the price. The longer the tube, the higher the price...though some sizes have very little cost differencs (ie 18" and 24" may only be a buck difference in price, or the same price. Yet if one tube is 5.0 and the other 10.0 and both the same size, there is a greater cost difference).
I can buy a 18" 8.0 UVB tube in Canada, for about $25.
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PHLdyPayne