I´m not familiar with leaving the rubsign exposed to higher temperatures for a longer period but the symbol normally appears between 79 and 85 degrees (according to the information from the card) which means that the 80+ degrees temps might indeed cause you´re Beasts to show their symbol permanently. Normally, if the temperature drops, the symbol will also disappear again but leaving it exposed to those temps for a week or more might´ve effect on the heat activated part of the rubsign but as mentioned before that´s not been experienced yet. Personally I´ve had my Beasts showing their symbols for a couple days in a row at max during the hot summer days and they´ve ALL been hiding it again as soon as the temperatures dropped so there´s been no harm done there. Based on this and the quality of the rubsigns, still working after almost 30 years, I doubt that damage might be done to them because of this but as some mentioned above the best way to check it is to hold it in the fridge for a little while because the change will appear very quickly. Of course things become different when the max temperature goes above a certain standard because they can´t handle hot water or extreme temps because that´ll do damage for sure. Let it know when you´ve done the ¨fridge test¨ and how long the 80+ degrees temps have been going on in your area as it might be interesting to know if and how the rubs react to those longer term exposure.
As far as pricing goes the prices of the complete figs are mainly composed out of the fact there the figure + rub + weapon and each part adds a certain amount of value building the total. The figure itself without rub, these days, goes anywhere from $1 (thrift store/flea market) to $5 (also depending upon condition) than a working rubsign normally adds around $2-3 to the figure meaning a figure in good condition with the rub ranges from $4-8 and the weapon adds another big chunk because those days, and with the ¨limited¨ amount of them around (most people are missing the weapons as they´ve gone missing or broke over time), these go for almost equal prices as the figure ranging from $4-7. This leads to a price of around $8-15 for a complete Battle Beast with weapon in good condition and with a working rub. Of course there´s a few exceptions because this doesn´t alway go for Series 2 and 3 Beasts as some are rated a bit higher but for most Series 1 and 2 Beasts this should be a good guideline at the moment.