A key item on this one is the peak voltage of "23.57" volts. This means it is into the solar power category and not the solar charger category (which maxes out at about 15 v). In other words, the panel would need a good charge controller to be truly useful. Cheap controllers for a panel this size can be had for under $50 but a good one is likely to be a bit over $100.
As for price comparison with https://secure.westhost.com/secure/r...rform.htm#Kits
as a reference, 45 watts for $200 is $4.44/watt versus $5.7/watt for an equiv size or $4.81/watt for a double that size. From this, it appears that the sale item is a fairly good value as far as solar energy is concerned. Sounds typical Harbor Freight.
45 watts will do to keep a battery charged during storage - if you have a good controller that knows about float and equalization. If might also work for very light use such as summertime needs for occasional lighting.
The rule of thumb for most RV purposes is that you need a minimum of a watt of solar power for each amp hour of battery you have and you need enough amp hour batteries to meet your needs for a typical weekend camp outing. For trailerists, this usually means 200 AH of batteries and 200 watts of solar panels.