Propane give about 92,000 BTU per gallon. This would be a little over 15 hours of heat per gallon of propane. You have to fit the propane bottle so that it is sealed off from the cockpit and vents overboard--probably converting one of the lazarettes to this function would work the best--but not sure what size propane bottle you can get in this space. I suspect that you can get a 10lb bottle [2.4 gallons] (horizontal) in one of those lockers. These are listed at $255 at West Marine. I have always used aluminum propane bottles in boats, because of corrosion--and availability of the horizontal bottles in steel painted bottles. Also to be ABYC compliant you will need a selenoid valve (and I feel that you also need a propane sniffer)--add in another $319. Not sure how long the hose is going to be, but figure with fittings that the cost will probably be about $5 a foot.
You probably will want some type of fan--to circulate the warm air--add another $40. So the cheaper heater, may not be as cheap as it seems.
Propane is a great fuel--I have used it on all of my cruising boats for almost 50 years. But you do have to have the shut off vavle, sniffer, proper installation etc. You do not want propane in the cabin or in the cockpit (propane is heavier than air and will pool in these places.). I have also personally seen two boat blow up with propane, and know of another two boats which were lost with propane, and have treated two people who had injuries from propane explosion--plus treated several others with CO poisoning from unvented heaters... so it is a fuel not to be taken lightly. Each individual has to decide if they want all of the safety features, or take a risk. I have always used the safety features with Propane, and for small boats felt that it was a bit of a hassel--thus the Wallas as a good alternative.
I happen to love the Wallas (mainly because I have been very lucky and had no problems with the three I have owned). It is vented to the outside, it has a fan, and it uses relitatively safe fuel. There are other kerosene/diesel heaters which might be a better choice (cost wise and only heaters) Toyo makes a boat heater, but I am not sure who carries them in the US. Taylor also makes diesel boat heaters, as does. Webasto used to make some small heaters (of course there is Hi seas and Dickenson). The problem with drip pot burners is that they can back draft, unless forced draft, such as the Wallas.