First, the transport. These are pretty good flight times in both directions, no problem at all with connections. Only small issue is the return journey at 1400, is that departure or check-in by? If it's departure you may need to leave Perth on the train just after 0900, if it's check-in you can use the one just after 1000. Allowing for connection time in Glasgow the overall journey Perth - Airport would be about 2 hours. Trains from Glasgow to Perth are at least every hour (every 30 minutes during the afternoon you arrive)
Prices: the return price on the airport bus to Queen St railway station would be £6.50 each if you buy the return ticket when you board at the airport. The return price on the train from Queen Street to Perth would be £22.60 each (open return) but if you could come back on the one after 1000 that falls to £13 (saver return). You need to know this before you buy the ticket on the first day. You don't have to make any advance reservation for the train but if you want to use the early one back it might be more convenient to make a reservation any time you're in Perth station because it could be busy (which is why it costs more!).
The £80 taxi price could be misleading. If you take a taxi from the rank at the airport that sounds like the one-way price but it might be possible to find a private hire service who would take a reservation for a round trip at that price. If the price didn't come with a name you should assume it's the rank price - which makes it £160 return compared with either £58.20, or possibly £39, return for two by public transport.
Getting around on trips shouldn't be too expensive, especially if you check what time the "off peak" train tickets become valid (around 0930 weekdays and anytime weekends). You can check prices via the journey planner at
http://www.nationalrail.co.uk , put in rough times for your trip and it will offer a selection of trains to match and offer to show prices, you may spot that cheap day/saver fares aren't marked against some busy trains and only open fares are given.
When checking hotels/guesthouses in Perth go for one with a postcode beginning PH1 or PH2 as these will be close to the centre. I've not stopped in Perth itself for years and British hotels tend to change ownership quite frequently so any recommendation from more than a couple of years ago may not be valid.
If I was looking for a place I'd start
here simply because it's an easy way to get a list. The prices on this page may not be 100% up to date but give a good comparison of how they vary - 4 star guesthouse are less than 3 star hotels. The difference between a guesthouse and hotel may be hard to spot, probably no permanent reception staff and no
alcohol licence (which is what SMa refered to - don't get the idea that they're not licenced for guests!!). Taking pot luck from the page I looked at this one and it does offer laundry but no mention of internet
http://www.dunallan.co.uk . The cheapest on the page doesn't offer either but will do you a double room with breakfasts, private bath and TV for £34 - so your budget looks safe!!
Guesthouses and B&Bs will always include full breakfast in the price (in Scotland this can be a
very full breakfast!). Most privately run hotels will include breakfast but some of the branded chains may not. Tea/coffee making faclities in the room (sometimes called Hospitality Trays and often with packets of shortbread) are very common and always included in the price.